Never said he needed it.
But G5 is leaps and bounds ahead of his other forms. G4 Snakeman got pummeled by Kizaru and hardly kept up with him in CQC, whereas G5 was easily able to spar with him.
So for Luffy to use G5 and for Lucci to not just withstand the clash, but take a few hits from it and recover soon after is just clear of anything and everything Cracker has done.
I'm genuinely asking, what has Cracker done to be on par with Lucci? Forget G5, I don't even think Cracker has a feat that's better than one-shotting Sentomaru