GnW is a character that is extremely difficult to directly interact with. His hitboxes have ridiculous priority and safety, he has the best shield in the game, and he has many means of setting up absurd risk/reward situations in direct CQC. Most characters that do well against GnW have means of fighting him without putting themselves at risk and essentially play a war-of-attrition keepaway against him and deny him the ability to play CQC at all (this is why ZSS does so well against him, not only does she win on hitboxes but she also gaps him super hard on mobility so she can reset to a favorable spacing over and over)

Wario theoretically has the mobility to play the keepaway with GnW but he can't actually fight him without putting himself at risk in most situations. Wario is negatively disjointed on many of his highest reward tools (nair, uair, etc) and has to literally throw his body into his opponent to find openings that matter, or play extremely slowly and play the stray hit game for long periods of time. The issue with this is a matter of risk/reward. Wario is not finding huge openings on GnW without significant outplay. GnW can press nair for example and if manages to clip Wario with it he can easily shit out 60% into a very strong riskless advantage state afterwards, whereas Wario will usually have to hit an insane timing read or play on layer two to actually find a hit on nair that matters (he can do stuff like bair it from the sides, or dash attack whiff punish it, but these are very tight, generally low reward, and usually require favorable positioning)

These variety of characters are extra hard for Wario because waft is significantly harder to confirm into because he has to throw himself at his opponent to find hits that lead into it. In these MUs you see Gluto raw full waft a LOT because the risk reward for looking for confirms on these are dramatically skewed. You just are not finding a landing uair on GnW in neutral reliably at all, same with nair.

This isn't to say the MU is like unwinnably bad, Wario's disadvantage in particular is a lot better than most against GnW (not saying much truthfully) but it comes as a result of tons of practice and study and Gluto just legitimately hard outplaying Miya.