You are right, it does not matter what is on the end of the machine repetier is just sending gcode including laser control, It can do spindle control but most people just turn it on and off by hand. That is how this machine is so easy to do all these different tasks.
GRBL doesn’t work on ramps boards as far as I know.
I think you will find it is no different, load the gcode, press start. That is all you need repetier for.