GPR is rare actually, so maxing it out of Vet packs is going to take awhile. (It's also generally not worth it - factoring in the reload times, the Avenger does more dps! Well, except at stupidly long ranges.)
I haven't ever used the Avenger. Is it as perfectly stable as the GPR? Because with a scope it's really, really easy to get a whole bunch of headshots and to stay on target with it.