I figured it out, at least for the mapping part. In my case I set my controller as controller 1 for navigating the menu and stuff, and G25 as Controller 2, so you need to change the DFP setting for controller 2 to get it to work. I am not sure though as to what button is used as brake bias. I assigned the Square as RA Function + and X as RA Function - but I don't know if I am suppose to get some kind of on screen notice if something got changed. Or should I have assigned it as RA Function Select +/-. I know the reassignment works because my reassigned high beam and wiper works.
Also do cars in GT5 have launch controls?