A shell script may improve your gyb experience a lot.

generate_boilerplates.sh

#!/bin/sh
# Scans all your *.swift.gyb files in your project folder and generates
#   your boilerplates.

# Known bugs:
# 1) Timestamp of files with the name contains +, - would conflict each 
#   other. This is caused by Bash 3.2 doesn't support associative array,
#   which enforces it to use dynamic variable name to store timestamps for
#   difference file names, and variable name in Bash script cannot 
#   contain characters like "+" and "-", so "+" and "-" are all converted
#   into "_".

# Usage:
# 1) Copy the gyb and gyb.py from the Swift's repository and put them in
#   Your_Project_Root_Folder/Utilities.
# 2) Add this shell script to your Xcode's build time phases.
# 3) Command + Shift + K(Clean build) would clean the boilerplates' 
#   creation timestamp, and you can use this key combination to enforce
#   the shell script to re-generate all the boilerplates.

SOURCE_DIR=$PROJECT_DIR
GYB="$PROJECT_DIR/Utilities/gyb"
TIMESTAMPS_LOC="$DERIVED_FILES_DIR/gyb_timestamps"

TIMESTAMP_KEYS=()

if [ -f $TIMESTAMPS_LOC ]; then
# Read timestamps
    while read -r LINE; do
        IFS=':' read -ra LINE_COMPONENTS <<< "$LINE"
        KEY="${LINE_COMPONENTS[0]}"
        TIMESTAMP="${LINE_COMPONENTS[1]}"
        declare TIMESTAMPS__${KEY}="$TIMESTAMP"
    done < "$TIMESTAMPS_LOC"
fi

IFS=$'\n'; for SOURCE_FILE in $(find /$SOURCE_DIR -name '*.gyb'); do
	if [[ "$SOURCE_FILE" =~ ^[^\0]+.swift.gyb$ ]]; then
        SOURCE_TIMESTAMP=`stat -f %m $SOURCE_FILE`
        KEY=${SOURCE_FILE//[.\/+\-\\]/_}
        TIMESTAMP_KEYS+=($KEY)
        TIMESTAMP_NAME=TIMESTAMPS__${KEY}
        DESTINATION_TIMESTAMP=${!TIMESTAMP_NAME}
        if [ -z "$DESTINATION_TIMESTAMP" ]; then
            declare DESTINATION_TIMESTAMP=0
        fi
        if [ $SOURCE_TIMESTAMP -ge $DESTINATION_TIMESTAMP ]; then
            DESTINATION_FILE=$(expr $SOURCE_FILE \
                | egrep -o "[^\0]+.swift")
            echo "Generate your biolerplate from $SOURCE_FILE to \
$DESTINATION_FILE"
            $GYB $SOURCE_FILE -o $DESTINATION_FILE --line-directive ""
            NEW_TIMESTAMP=`stat -f %m $DESTINATION_FILE`
            declare TIMESTAMPS__${KEY}="$NEW_TIMESTAMP"
            EXIT_CODE=$?
            if [[ $EXIT_CODE != 0 ]]; then
                exit $EXIT_CODE
            fi
        else
            echo "Generated biolerplate found: $DESTINATION. \
Re-generation was skipped due to the template's last modified \
timestamp is ealier than the generated biolerplate's."
        fi
	fi
done

> "$TIMESTAMPS_LOC"

for KEY in "${TIMESTAMP_KEYS[@]}"; do
    TIMESTAMP_NAME=TIMESTAMPS__${KEY}
    echo "$KEY:${!TIMESTAMP_NAME}" >> "$TIMESTAMPS_LOC"
done

exit 0