That’s strange they had you use QMK to try and revert your board if you never had QMK on it to begin with. Official firmware should be obtainable via Glorious CORE. Have you tried giving it an update through there? It should detect your board. How long have you had it? Could potentially RMA or get a replacement PCB. The support team has been questionable lately.
If it is detected just go down to Settings > Reinstall Firmware > GMMK PRO
