Gearbox version corresponds to the "shape" of the gearbox. So M4's and MP5's will use V2 gearbox, AK and AUG will use V3 that's basically all you need to concern yourself with as it's the most common.
Less common guns like the FAMAS will use a V1, the P90 uses a V6 and the M14/M1A uses a V7 design.
I don't have that great of an opinion of Action, to me it's a brand that you get from places like EBaybanned and the like, G&P is good though (I have one).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE