Ask the program director/attending docs questions about the core layout of the program (rotations, call requirements, continuing ed opportunities, etc).
Ask the current residents what it's REALLY like (number + types of procedures you'll be doing, what call is REALLY like, etc)
Bottomline, for someone to get the most out of a GPR as possible, I fully feel that after your interview there, you should be asking yourself "can I really handle all this" instead of "this seems like it will be easy" A GPR is a great chance to really push your developing skills in a still controlled environment before you hit private practice. So you want to find a program that WILL more than likely push you past your comfort level, as opposed to a program where you'll likely just be cruising along at your current comfort level