blender python ui gizmo POC, does NOT work with custom icons

poc_2d_gizmos.py

from bpy.types import (
    GizmoGroup
)
import bpy

bl_info = {
    "name": "POC buttons",
    "description": "POC 2D BUTTON script refactored from GP Animator Desk addon",
    "author": "Anthony Aragues, P.Świda",
    "version": (0, 0, 1),
    "blender": (3, 4, 0),
    "location": "3d window",
    "wiki_url": "",
    "tracker_url": "",
    "category": "UI"
}


class POC_UI_Buttons(GizmoGroup):
    bl_idname = "POC UI Buttons"
    bl_label = "POC UI Buttons"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'WINDOW'
    bl_options = {'PERSISTENT', 'SCALE'}

    @classmethod
    def poll(cls, context):
        # modify this to whatever conditions you need for showing this gizmo group
        return True

    def draw_prepare(self, context):
        # set the x and y for the gizmo, by default starting from bottom left == the center of the gizmo
        intGizmoSize = 70
        intTotalSize = 0
        intGizmoCount = 0
        for strGizmo in self.objGizmos:
            intGizmoCount += 1
            intTotalSize += intGizmoSize
        for i, strGizmo in enumerate(self.objGizmos):
            objGizmo = getattr(self, strGizmo)
            intX = (context.region.width/2) - \
                (intTotalSize/2) + (intGizmoSize*i)
            objGizmo.matrix_basis[0][3] = intX
            objGizmo.matrix_basis[1][3] = 43
        return

    def setup(self, context):
        objGizmoDefaults = {
            "color": (1, 0, 0),
            "color_highlight": (0, 0, 1),
            "show_drag": False,
            "scale_basis": 15.5,
            "alpha": 0.8,
            "draw_options": {'BACKDROP', 'OUTLINE'},
            "icon": 'BLENDER',
            "use_tooltip": True
        }
        self.objGizmos = {
            "insert_btn": {
                "icon": 'FUND'
            },
            "remove_btn": {
                "icon": 'COPY'
            }
        }

        for strGizmo in self.objGizmos:
            # create the new gizmo
            objGizmo = self.gizmos.new("GIZMO_GT_button_2d")
            # start it with some defaults
            for strProp in objGizmoDefaults:
                setattr(objGizmo, strProp, objGizmoDefaults[strProp])
            # override the defaults
            for strProp in self.objGizmos[strGizmo]:
                setattr(objGizmo, strProp, self.objGizmos[strGizmo][strProp])
            # attach the gizmo to 'self'
            setattr(self, strGizmo, objGizmo)


def register():
    bpy.utils.register_class(POC_UI_Buttons)


def unregister():
    bpy.utils.unregister_class(POC_UI_Buttons)