I test drove 3 04+ Grand Prix's before settling on my 02, and all of the 04+ grand prix's studdered a bit when starting, and on two of them you had to give it a little gas to keep them going when starting. After they got running it was fine.
I always suggest cleaning the throttle body and cleaning the MAF sensor (each have their own specific cleaner) at the same time and seeing if that fixes the issue. If not, tuneup items should be looked over/replaced.