Few thoughts . . .
I. Gohan's assessment seems fair to me. For starters, Gohan's not really saying SSJ4>Ultra Instinct because his frame of reference would be Goku as SSJ Blue & Ultra Instinct. Gohan wasn't around for SSJ Blue Gogeta and Ultra Instinct Gogeta can't exist. Fusion Dance power increase is a multiplier right? That's how I remember it, I can believe with the fusion multiplier two SSJ4's might reach heights comparable, it would just mean SSJ Blue Gogeta is that much ahead. But I also didn't watch enough GT to really know how it scales. But basically, I don't take it as Gohan's statement as saying the transformation of SSJ4 is inherently stronger than Blue or Ultra Instinct, that would be dumb. But even so, that would be Gohan's opinion, and Gohan's opinion is that the Great Saiyaman is cool don't take him so seriously fanboys.
II. The game has dramatic finishes for SSJ4 Gogeta vs. SSJ Blue Gogeta. Depending on who wins the fight, depends on how it plays out, with one of the Gogeta's proudly claiming their transformation is superior you idiot other Gogeta. Feats beat statements, and this game have feats that argue for both of them because the point is to please fans of bth. They use to do with Vegito vs. Gogeta, I think it was Raging Blast that had two separate missions where you fought one as the other, with a funny cutscene afterward. By the way, the cutscenes were awesome. If Gogeta won, Vegito asks for a rematch but Gogeta can't because his fusion's almost up so Vegito claims to be the victor by default. If Vegito wins, iirc he ends up getting upset by the fact that he's fused together forever while Gogeta can separate and go back to their families and stuff. But to echo everyone else, my point here is that they're non-canon and catering to both because hey, it's non-canon, have the canon you want.
III. There can never be a canon answer to this. GT isn't canon, SSJ4 isn't Toriyama's creation. It's like asking, who would win in a fight, Luffy or Naruto and expecting an answer from Oda to be the definitive truth on the matter. If the Super manga ever adds in SSJ4, it won't be the same as the SSJ4 in GT, it would be Toriyama's version of SSJ4. Much in the same way the Super Broly is not Z Broly. If Toriyama claimed the SSJ Blue was stronger than SSJ4 from GT, I mean, looked I'd agree with him but that's still him assessing his opinion on somebody else's work. It's almost like asking George Lucas if Rey is better at the force than Luke, he's the creator of Star Wars but he wasn't the one who wrote Rey's journey so he doesn't have full authority on this.