Patch for QMK firmware to add two keycodes for Apple Fn: KC_APFN simulates the key directly, whereas APFN_LAYER(x) works as MO(x) but also registers as holding down Apple Fn _when no other key is pressed_. APFN_KEY(x) is key x together with Apple Fn.

qmk_apple_fn.patch

diff --git a/builddefs/common_features.mk b/builddefs/common_features.mk
index c976b8296d..335b432e78 100644
--- a/builddefs/common_features.mk
+++ b/builddefs/common_features.mk
@@ -108,6 +108,12 @@ ifeq ($(strip $(MOUSEKEY_ENABLE)), yes)
     SRC += $(QUANTUM_DIR)/mousekey.c
 endif
 
+ifeq ($(strip $(APPLE_FN_ENABLE)), yes)
+    OPT_DEFS += -DAPPLE_FN_ENABLE -DENABLE_APPLE_FN_KEY=1
+else
+    OPT_DEFS += -DENABLE_APPLE_FN_KEY=0
+endif
+
 VALID_POINTING_DEVICE_DRIVER_TYPES := adns5050 adns9800 analog_joystick cirque_pinnacle_i2c cirque_pinnacle_spi pmw3360 pmw3389 pimoroni_trackball custom
 ifeq ($(strip $(POINTING_DEVICE_ENABLE)), yes)
     ifeq ($(filter $(POINTING_DEVICE_DRIVER),$(VALID_POINTING_DEVICE_DRIVER_TYPES)),)
diff --git a/keyboards/gmmk/pro/config.h b/keyboards/gmmk/pro/config.h
index aba69c8872..aa1a262d90 100644
--- a/keyboards/gmmk/pro/config.h
+++ b/keyboards/gmmk/pro/config.h
@@ -20,8 +20,13 @@
 
 /* USB Device descriptor parameter */
 #define DEVICE_VER 0x0001
+#if ENABLE_APPLE_FN_KEY
+#define VENDOR_ID 0x05AC
+#define PRODUCT_ID 0x021E
+#else
 #define VENDOR_ID 0x320F
 #define PRODUCT_ID 0x5044
+#endif
 #define MANUFACTURER Glorious
 #define PRODUCT GMMK Pro
 
diff --git a/keyboards/gmmk/pro/rev1/iso/rules.mk b/keyboards/gmmk/pro/rev1/iso/rules.mk
index d3afa4d432..fccc7035b6 100644
--- a/keyboards/gmmk/pro/rev1/iso/rules.mk
+++ b/keyboards/gmmk/pro/rev1/iso/rules.mk
@@ -8,14 +8,15 @@ BOOTLOADER = stm32-dfu
 #   change yes to no to disable
 #
 BOOTMAGIC_ENABLE = yes      # Enable Bootmagic Lite
-MOUSEKEY_ENABLE = yes       # Mouse keys
+MOUSEKEY_ENABLE = no        # Mouse keys
 EXTRAKEY_ENABLE = yes       # Audio control and System control
 CONSOLE_ENABLE = no         # Console for debug
 COMMAND_ENABLE = no         # Commands for debug and configuration
-NKRO_ENABLE = yes           # Enable N-Key Rollover
+NKRO_ENABLE = no            # Enable N-Key Rollover
 BACKLIGHT_ENABLE = no       # Enable keyboard backlight functionality
 RGBLIGHT_ENABLE = no        # Enable keyboard RGB underglow
 AUDIO_ENABLE = no           # Audio output
 ENCODER_ENABLE = yes
 RGB_MATRIX_ENABLE = yes
 RGB_MATRIX_DRIVER = AW20216
+APPLE_FN_ENABLE = yes
diff --git a/quantum/action.c b/quantum/action.c
index 4e81a5466f..cc6e53c102 100644
--- a/quantum/action.c
+++ b/quantum/action.c
@@ -306,6 +306,10 @@ void register_button(bool pressed, enum mouse_buttons button) {
 }
 #endif
 
+#if ENABLE_APPLE_FN_KEY
+static bool virtual_apple_fn_on = false;
+#endif
+
 /** \brief Take an action and processes it.
  *
  * FIXME: Needs documentation.
@@ -585,6 +589,53 @@ void process_action(keyrecord_t *record, action_t action) {
                 layer_off(action.layer_mods.layer);
             }
             break;
+#if ENABLE_APPLE_FN_KEY
+        case ACT_APPLE_FN:
+            if (event.pressed) {
+                if (action.key.code) {
+                    if (!virtual_apple_fn_on) {
+                        add_key(KC_APPLE_FN);
+                        send_keyboard_report();
+                        virtual_apple_fn_on = true;
+                    }
+                    add_key(action.key.code);
+                    send_keyboard_report();
+                } else {
+                    virtual_apple_fn_on = false;
+                    register_code(KC_APPLE_FN);
+                }
+            } else {
+                if (action.key.code) {
+                    del_key(action.key.code);
+                    if (virtual_apple_fn_on) {
+                        del_key(KC_APPLE_FN);
+                        virtual_apple_fn_on = false;
+                    }
+                    send_keyboard_report();
+                } else {
+                    unregister_code(KC_APPLE_FN);
+                }
+            }
+            break;
+#ifdef NO_ACTION_TAPPING
+        case ACT_LAYER_TAP:
+        case ACT_LAYER_TAP_EXT:
+            if (action.layer_tap.code == OP_APPLE_FN) {
+                if (event.pressed) {
+                    layer_on(action.layer_tap.val);
+                    register_code(KC_APPLE_FN);
+                    virtual_apple_fn_on = true;
+                } else {
+                    if (virtual_apple_fn_on) {
+                        virtual_apple_fn_on = false;
+                        unregister_code(KC_APPLE_FN);
+                    }
+                    layer_off(action.layer_tap.val);
+                }
+            }
+            break;
+#endif
+#endif
 #    ifndef NO_ACTION_TAPPING
         case ACT_LAYER_TAP:
         case ACT_LAYER_TAP_EXT:
@@ -653,6 +704,21 @@ void process_action(keyrecord_t *record, action_t action) {
                     }
                     break;
 #        endif
+#if ENABLE_APPLE_FN_KEY
+                case OP_APPLE_FN:
+                    if (event.pressed) {
+                        layer_on(action.layer_tap.val);
+                        register_code(KC_APPLE_FN);
+                        virtual_apple_fn_on = true;
+                    } else {
+                        if (virtual_apple_fn_on) {
+                            virtual_apple_fn_on = false;
+                            unregister_code(KC_APPLE_FN);
+                        }
+                        layer_off(action.layer_tap.val);
+                    }
+                    break;
+#endif
                 default:
                     /* tap key */
                     if (event.pressed) {
@@ -892,6 +958,13 @@ __attribute__((weak)) void register_code(uint8_t code) {
 */
 #endif
         {
+#if ENABLE_APPLE_FN_KEY
+            if (virtual_apple_fn_on) {
+                del_key(KC_APPLE_FN);
+                send_keyboard_report();
+                virtual_apple_fn_on = false;
+            }
+#endif
             // Force a new key press if the key is already pressed
             // without this, keys with the same keycode, but different
             // modifiers will be reported incorrectly, see issue #1708
@@ -919,6 +992,12 @@ __attribute__((weak)) void register_code(uint8_t code) {
         mousekey_send();
     }
 #endif
+#if ENABLE_APPLE_FN_KEY
+    else if (code == KC_APPLE_FN) {
+        add_key(code);
+        send_keyboard_report();
+    }
+#endif
 }
 
 /** \brief Utilities for actions. (FIXME: Needs better description)
@@ -979,6 +1058,12 @@ __attribute__((weak)) void unregister_code(uint8_t code) {
         mousekey_send();
     }
 #endif
+#if ENABLE_APPLE_FN_KEY
+    else if (code == KC_APPLE_FN) {
+        del_key(code);
+        send_keyboard_report();
+    }
+#endif
 }
 
 /** \brief Tap a keycode with a delay.
diff --git a/quantum/action_code.h b/quantum/action_code.h
index 20b3e459d2..4409ac4618 100644
--- a/quantum/action_code.h
+++ b/quantum/action_code.h
@@ -51,6 +51,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
  * ACT_SWAP_HANDS(0110):
  * 0110|xxxx| keycode     Swap hands (keycode on tap, or options)
  *
+ * 0111|0001 xxxx xxxx    Apple Fn + key
  * 0111|xxxx xxxx xxxx    (reserved)
  *
  * Layer Actions(10xx)
@@ -77,8 +78,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
  * 101E|LLLL|1111 0010   Off/On                 (0xF2)   [NOT TAP]
  * 101E|LLLL|1111 0011   Set/Clear              (0xF3)   [NOT TAP]
  * 101E|LLLL|1111 0100   One Shot Layer         (0xF4)   [TAP]
- * 101E|LLLL|1111 xxxx   Reserved               (0xF5-FF)
- *   ELLLL: layer 0-31(E: extra bit for layer 16-31)
+ * 101E|LLLL|1111 0101   Apple Fn Layer         (0xF5)
+ * 101E|LLLL|1111 xxxx   Reserved               (0xF6-FF)
  */
 enum action_kind_id {
     /* Key Actions */
@@ -98,6 +99,9 @@ enum action_kind_id {
     ACT_LAYER_MODS    = 0b1001,
     ACT_LAYER_TAP     = 0b1010, /* Layer  0-15 */
     ACT_LAYER_TAP_EXT = 0b1011, /* Layer 16-31 */
+#if ENABLE_APPLE_FN_KEY
+    ACT_APPLE_FN = 0b0111
+#endif
 };
 
 /** \brief Action Code Struct
@@ -222,6 +226,9 @@ enum layer_param_tap_op {
     OP_OFF_ON,
     OP_SET_CLEAR,
     OP_ONESHOT,
+#if ENABLE_APPLE_FN_KEY
+    OP_APPLE_FN
+#endif
 };
 #define ACTION_LAYER_BITOP(op, part, bits, on) ACTION(ACT_LAYER, (op) << 10 | (on) << 8 | (part) << 5 | ((bits)&0x1f))
 #define ACTION_LAYER_TAP(layer, key) ACTION(ACT_LAYER_TAP, (layer) << 8 | (key))
@@ -254,6 +261,12 @@ enum layer_param_tap_op {
 #define ACTION_DEFAULT_LAYER_BIT_XOR(part, bits) ACTION_LAYER_BITOP(OP_BIT_XOR, (part), (bits), 0)
 #define ACTION_DEFAULT_LAYER_BIT_SET(part, bits) ACTION_LAYER_BITOP(OP_BIT_SET, (part), (bits), 0)
 
+#if ENABLE_APPLE_FN_KEY
+#define ACTION_APPLE_FN_KEY(key)        ACTION(ACT_APPLE_FN, (key))
+#define ACTION_APPLE_FN()               ACTION_APPLE_FN_KEY(0)
+#define ACTION_APPLE_FN_LAYER(layer)    ACTION_LAYER_TAP((layer), OP_APPLE_FN)
+#endif
+
 /* OneHand Support */
 enum swap_hands_param_tap_op {
     OP_SH_TOGGLE = 0xF0,
diff --git a/quantum/keycode.h b/quantum/keycode.h
index 3c80a386d1..762b1bf73f 100644
--- a/quantum/keycode.h
+++ b/quantum/keycode.h
@@ -502,9 +502,17 @@ enum internal_special_keycodes {
     KC_MEDIA_FAST_FORWARD,
     KC_MEDIA_REWIND,
     KC_BRIGHTNESS_UP,
-    KC_BRIGHTNESS_DOWN
+    KC_BRIGHTNESS_DOWN,
+
+#if ENABLE_APPLE_FN_KEY
+    KC_APFN,
+#endif
 };
 
+#if ENABLE_APPLE_FN_KEY
+#define KC_APPLE_FN KC_APFN
+#endif
+
 enum mouse_keys {
 /* Mouse Buttons */
 #ifdef VIA_ENABLE
diff --git a/quantum/keymap_common.c b/quantum/keymap_common.c
index c1940f0fd3..efda95d52c 100644
--- a/quantum/keymap_common.c
+++ b/quantum/keymap_common.c
@@ -58,6 +58,17 @@ action_t action_for_keycode(uint16_t keycode) {
         case KC_LEFT_CTRL ... KC_RIGHT_GUI:
             action.code = ACTION_KEY(keycode);
             break;
+#if ENABLE_APPLE_FN_KEY
+        case KC_APPLE_FN:
+            action.code = ACTION_APPLE_FN();
+            break;
+        case QK_APPLE_FN_MOD ... QK_APPLE_FN_MOD_MAX:
+            action.code = ACTION_APPLE_FN_KEY(keycode & 0xFF);
+            break;
+        case QK_APPLE_FN ... QK_APPLE_FN_MAX:
+            action.code = ACTION_APPLE_FN_LAYER(keycode & 0x1F);
+            break;
+#endif
 #ifdef EXTRAKEY_ENABLE
         case KC_SYSTEM_POWER ... KC_SYSTEM_WAKE:
             action.code = ACTION_USAGE_SYSTEM(KEYCODE2SYSTEM(keycode));
diff --git a/quantum/quantum_keycodes.h b/quantum/quantum_keycodes.h
index 40355d799a..c99a814cf6 100644
--- a/quantum/quantum_keycodes.h
+++ b/quantum/quantum_keycodes.h
@@ -37,6 +37,12 @@ enum quantum_keycodes {
     QK_RALT                 = 0x1400,
     QK_RGUI                 = 0x1800,
     QK_MODS_MAX             = 0x1FFF,
+#if ENABLE_APPLE_FN_KEY
+    QK_APPLE_FN_MOD         = 0x3E00,
+    QK_APPLE_FN_MOD_MAX     = 0x3EFF,
+    QK_APPLE_FN             = 0x3FE0,
+    QK_APPLE_FN_MAX         = 0x3FFF,
+#endif
     QK_LAYER_TAP            = 0x4000,
     QK_LAYER_TAP_MAX        = 0x4FFF,
     QK_TO                   = 0x5000,
@@ -823,6 +829,11 @@ enum quantum_keycodes {
 // L-ayer, T-ap - 256 keycode max, 16 layer max
 #define LT(layer, kc) (QK_LAYER_TAP | (((layer)&0xF) << 8) | ((kc)&0xFF))
 
+#if ENABLE_APPLE_FN_KEY
+#define APFN_LAYER(layer)   (QK_APPLE_FN | ((layer) & 0x1F))
+#define APFN_KEY(key)       (QK_APPLE_FN_MOD | ((key) & 0xFF))
+#endif
+
 // M-od, T-ap - 256 keycode max
 #define MT(mod, kc) (QK_MOD_TAP | (((mod)&0x1F) << 8) | ((kc)&0xFF))
 
diff --git a/tmk_core/protocol/report.c b/tmk_core/protocol/report.c
index 5755098c60..ab596b603e 100644
--- a/tmk_core/protocol/report.c
+++ b/tmk_core/protocol/report.c
@@ -241,6 +241,12 @@ void del_key_bit(report_keyboard_t* keyboard_report, uint8_t code) {
  * FIXME: Needs doc
  */
 void add_key_to_report(report_keyboard_t* keyboard_report, uint8_t key) {
+#if ENABLE_APPLE_FN_KEY
+    if (key == KC_APPLE_FN) {
+        keyboard_report->reserved = 1;
+        return;
+    }
+#endif
 #ifdef NKRO_ENABLE
     if (keyboard_protocol && keymap_config.nkro) {
         add_key_bit(keyboard_report, key);
@@ -255,6 +261,12 @@ void add_key_to_report(report_keyboard_t* keyboard_report, uint8_t key) {
  * FIXME: Needs doc
  */
 void del_key_from_report(report_keyboard_t* keyboard_report, uint8_t key) {
+#if ENABLE_APPLE_FN_KEY
+    if (key == KC_APFN) {
+        keyboard_report->reserved = 0;
+        return;
+    }
+#endif
 #ifdef NKRO_ENABLE
     if (keyboard_protocol && keymap_config.nkro) {
         del_key_bit(keyboard_report, key);
diff --git a/tmk_core/protocol/usb_descriptor.c b/tmk_core/protocol/usb_descriptor.c
index 063bd2c3f1..a7fc5672b6 100644
--- a/tmk_core/protocol/usb_descriptor.c
+++ b/tmk_core/protocol/usb_descriptor.c
@@ -71,10 +71,22 @@ const USB_Descriptor_HIDReport_Datatype_t PROGMEM KeyboardReport[] = {
         HID_RI_REPORT_COUNT(8, 0x08),
         HID_RI_REPORT_SIZE(8, 0x01),
         HID_RI_INPUT(8, HID_IOF_DATA | HID_IOF_VARIABLE | HID_IOF_ABSOLUTE),
+
+#if ENABLE_APPLE_FN_KEY
+        HID_RI_USAGE_PAGE(8, 0xFF),    // AppleVendor Top Case
+        HID_RI_USAGE(8, 0x03),         // KeyboardFn
+        HID_RI_LOGICAL_MINIMUM(8, 0x00),
+        HID_RI_LOGICAL_MAXIMUM(8, 0x01),
+        HID_RI_REPORT_COUNT(8, 0x01),
+        HID_RI_REPORT_SIZE(8, 0x08),
+        HID_RI_INPUT(8, HID_IOF_DATA | HID_IOF_VARIABLE | HID_IOF_ABSOLUTE),
+#else
         // Reserved (1 byte)
         HID_RI_REPORT_COUNT(8, 0x01),
         HID_RI_REPORT_SIZE(8, 0x08),
         HID_RI_INPUT(8, HID_IOF_CONSTANT),
+#endif
+
         // Keycodes (6 bytes)
         HID_RI_USAGE_PAGE(8, 0x07),    // Keyboard/Keypad
         HID_RI_USAGE_MINIMUM(8, 0x00),

I just tried to merge qmk master into your apple_fn branch, there's only 3 files that conflict but they conflict in ways that I don't understand (something like 5k files changed,phew!). Like I can't find where:

#define KC_INT6 KC_INTERNATIONAL_6
#define KC_INT7 KC_INTERNATIONAL_7
#define KC_INT8 KC_INTERNATIONAL_8
#define KC_INT9 KC_INTERNATIONAL_9
#define KC_LNG1 KC_LANGUAGE_1
#define KC_LNG2 KC_LANGUAGE_2
#define KC_LNG3 KC_LANGUAGE_3
#define KC_LNG4 KC_LANGUAGE_4
#define KC_LNG5 KC_LANGUAGE_5
#define KC_LNG6 KC_LANGUAGE_6
#define KC_LNG7 KC_LANGUAGE_7
#define KC_LNG8 KC_LANGUAGE_8

This block of keycode.h is now gone? as well as a large chunk of quantum_keycodes.h has disappeared.