SRC Gen III guns outclass G&G on the specs chart by a mile.
Heres a look at the SRC Gen III M4 gearbox, its just great. It would be similar to the G36's internals in quality.
If you want to go a bit cheaper you could get the SRC GEN II XM8. Which was the best XM8 on the market back in GEN II, and is the best SRC GEN II gun.