Luffy doesn't start his battles with g4/g5
If he does, it's to show off like vs the beast porstes fodder on the sea alongside Kidd and Law or numbers or g5 vs lucci
He uses G4/g5 when the opponent is too strong and he has no choice like vs doffy, Kata, Cracker, kaido and soon Kizaru
I guess Kizaru is weaker than Lucci since Luffy didn't use g4/g5 yet from the jump against him