Apparently Gumball would get Johnny to stop trying to attack him by writing it down in the Magic Notebook, and he would have everything cancel out with the Helmet, even though:
1) Gumball has to...well, write something down. Johnny won't stand there like an idiot, plus in the episode the notebook was featured in, it was shown Gumball kinda sucked with using it.
And 2) Gumball with the Helmet may be a formidable opponent, but it's still important to remember it can get knocked off his head with ranged attacks. Lasers and everything else Johnny has >>> Darwin and Anais throwing mud at him. So with that, combined with the AP gap and Johnny's ninja DNA, the moment Johnny hits Gumball, he's as good as gone