Gremmy could imagine that he's resistant to heat or fireproof. But if Gremmy is in character (aka stupid or limited in his imagination), then Yama wins with East or West. If he's anything like the Kenpachi fight, Gremmy would try to tank the attack or make himself 'fireproof', but then he'd get erased from existence by Yama. But, if Gremmy imagines himself as say...resistant to heat, then Yama might have some trouble. I don't think the meteor would be an issue for Yama, or really anything that Gremmy threw at Kenpachi besides Galaxy Room, which idk if Yama has an answer for. Meanwhile, aside from his Zan abilities, Yama got mastered Zanjutsu, Hakuda, Hoho, and Kido, but Gremmy could more than likely find ways around them.
So to me, it depends on if Gremmy properly defends Yama's existence erasure Bankai or not. There's also the argument that Yama can also be a Reiatsu monster like Kenpachi was and scare Gremmy into panicking or killing himself, since Yhwach said he's the only Quincy that could handle Yama's Bankai, and this was before the Elite received their amps, so Gremmy doesn't necessarily scale to all of them per say.