Gun Owners of America filed an amicus, friend of the court brief. This simply supplements Gura's brief. I don't think it really makes a difference. It presents a slightly different twist on arguments already made, based on my quick scan. Most organizations file them to look good for their members, but the court rarely cares about at them. The court will look at the ones filed by the fed gov, or if the group that files it presents an unusual perspective (i.e. police firearms instructors), but I doubt the GOA brief will come up.
The NRA is a co-petitioner, since their case was merged on appeal in the 7th cir., however their case was not selected by SCOTUS, Gura's was. This allowed them to hijack 1/3 of Gura's oral argument time.
Gura is pissed that the NRA tries to hijack every case he brings (Including Heller). GOA didn't do anything close to this.