qmk_firmware/keyboards/blackplum/keymaps/default/keymap.c

#include QMK_KEYBOARD_H

// Each layer gets a name for readability, which is then used in the keymap matrix below.
// The underscores don't mean anything - you can have a layer called STUFF or any other name.
// Layer names don't all need to be of the same length, obviously, and you can also skip them
// entirely and just use numbers.
#define _BASE 0
#define _FN 1



enum custom_keycodes {
  TG_GUI = SAFE_RANGE
};

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
  [_BASE] = LAYOUT_68_ansi(
    KC_GESC, 	KC_1,    	KC_2,    	KC_3,    KC_4,    	KC_5,    KC_6,    KC_7,    	KC_8,    	KC_9,    	KC_0,    	KC_MINS, 	KC_EQL,  	KC_BSPC, 	KC_INS,		KC_PGUP,
    KC_TAB,  	KC_Q,    	KC_W,    	KC_E,    KC_R,    	KC_T,    KC_Y,    KC_U,    	KC_I,    	KC_O,    	KC_P,    	KC_LBRC, 	KC_RBRC, 	KC_BSLS, 	KC_DEL,		KC_PGDOWN,
    KC_CAPS, 	KC_A,    	KC_S,    	KC_D,    KC_F,    	KC_G,    KC_H,    KC_J,    	KC_K,    	KC_L,    	KC_SCLN, 	KC_QUOT, 				KC_ENT,
    KC_LSFT, 	KC_Z,    	KC_X,    	KC_C,    KC_V,    	KC_B,    KC_N,    KC_M,    	KC_COMM, 				KC_DOT,  	KC_SLSH, 				KC_RSFT,	KC_UP,
    KC_LCTL , 	KC_LGUI , 	KC_LALT ,                      	KC_SPC ,											KC_RALT , 	MO(_FN) ,	KC_RCTL, 	KC_LEFT,	KC_DOWN,	KC_RIGHT
  ),

  [_FN] = LAYOUT_68_ansi(
    KC_GRV, 	KC_F1,   	KC_F2,   	KC_F3,   KC_F4,   	KC_F5,   KC_F6,   KC_F7,   	KC_F8,   	KC_F9,   	KC_F10,  	KC_F11,  	KC_F12,  	_______,	KC_MPLY,	KC_HOME,
    _______, 	RGB_MOD, 	RGB_HUI,   	RGB_SAI, RGB_VAI, 	_______, _______, _______, 	_______, 	_______, 	KC_PSCR, 	KC_SLCK, 	KC_PAUSE,  	_______,	KC_MUTE,	KC_END,
    _______,	RGB_TOG, 	RGB_HUD, 	RGB_SAD, RGB_VAD, 	_______, _______, _______, 	_______, 	_______, 	_______, 	_______, 				_______,
    _______,	_______, 	_______, 	_______, _______, 	_______, _______, _______,	_______, 				_______, 	_______, 				_______,	KC_VOLU,
    _______, 	TG_GUI,_______, 						RESET, 												_______,	_______,	_______,	KC_MPRV,    KC_VOLD, 	KC_MNXT
  )
};

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    if(record->event.pressed) {
        switch(keycode) {
            case TG_GUI:
                keymap_config.no_gui ^= 1;
                return false;
        }
    }
    return true;
}