You can give Gumball the Phone and Console if you want to, since I debunked those items being 4-A/3-A.
Anyway, Johnny should take this. Sure, the Helmet makes Gumball a formidable opponent, but
it can still get knocked off with ranged attacks, which is what Johnny has plenty of, mostly with lasers. Lasers are much superior to Darwin and Anais throwing mud, so with that, combined with the massive AP gap, Gumball turns into a roasted turkey once a laser hits him. But would Johnny even need lasers? Johnny could
wipe out Gumball's memories,
put him to sleep or
simply get him to stop attacking him, and Johnny's far more prone to utilizing his hax than Gumball. It's practically the point of the show anyway: he gets his hands on Susan and Mary's inventions to help him out with his life, and then usually by the episode's climax, he'll have to use them against an enemy.
Speaking of the AP gap, Gumball's as good as gone once the Helmet is knocked off of him, not just because of AP, but also because of skill/experience. Johnny regularly fights on par with Dukey, who has ninja DNA, and Johnny himself is a black belt, so Gumball would get outskilled. Gumball's first tactic in any fight is to run away from his opponent (there's 10 examples on his profile, and I can guarantee that's not even all of them), so...yeah. I'm not denying Gumball can fight (he wrestled with Rob after he ruined his life, and he literally beat up a group of kids in
The Faith), but it's far more consistent and more in his character to just run, and even then, Gumball is no black belt, and he'd still get one-shot.
As for the rest of his equipment, the Magic Notebook takes time to form an attack (he has to write, plus the episode showed Gumball kinda sucked at using it), so if Gumball runs away and uses it (or just starts writing something down in general), Johnny ain't letting that slide. The Love Arrows can easily be dodged, and the paintball gun will only temporarily blind Johnny at best.
Gumball's only true shots are with the Ghost Potion and the pocket watch, but Johnny has Non-Physical Interaction. However, Johnny has no answer to getting possessed, so he'd have to be quick.
