Let him be happy

luffy wasn't even taking him seriously while Gaban used haki and named attack
Luffy (no haki, no gears)
Even when he used g5, he didn't coat himself with Coa or CoC
Gaban knew all along he wouldn't stand a chance against serious Luffy
He just came to see G5 and make sure Luffy can be trusted per what Shanks told him