The GRBL you have to upload to the Arduino Mega is GRBL firmware. For that board I modified the code. You can see the code in this commit of github: https://github.com/AngelLM/grbl/tree/a4ab72a57b4f2c84cec56b8ef5326d89aad7727d
Also, for the GUI you can check this video I uploaded days ago 🙂 https://www.youtube.com/watch?v=Aas1Xh42xMA
I’ll upload soon a step to step guide to install and run the GUI.