What the hell? Gallit gun is NOT weak. Watch the Buu saga, the little ghosts fire it at Veggito. The kamehameha and the GG were the same strength fired from the little buu ghosts.
He doesnt use the gallit gun only once, either. During frieza saga, when Vegeta powered up against frieza, Vegeta went up and charged a purple beam(in his aura), and put his hands out in front of him in GG style firing, and the beam was very huge and purple. he just doenst say "gallit gun fire."(this is the only other documented gg)
Where the hell do you get the GG is weak anyways?? goku's power level at Kaioken times 4 was 36,000, and vegeta's was still 21,000. at kaioken times 3, Goku's pl was 22,000, and vegeta's was 21,000 (pretty much a deadlock ps). It wasnt the beam's strength, it was the fact that goku used the kaioken technique to boost his power temporarily to overcome the attack.