im hopeing your friend used a wiring harness to install it and not just doing a hack job on the wires. you should be using 1 of 2 possible harnesses, 1 will just have a chime module built into it since the door chime comes through the stereo. the other 1 will allow you to use your onstar through it still and have the chime module.
sorry i cant help on what the wireing colors are supposed to be on the gp as i havent installed an aftermarket in my car yet. but i have installed an aftermarket stereo in my wifes 05 cavy which is setup the same -onstar it required the GMCO wiring harness.
if your friend already did the hack job on the wires your best bet would be to get a wiring diagram for the stereo wires. there may be 1 online somewheres or you can usually get a copy of it from your local dealership