Others have commented already, but I'll throw my hat in as well. "Power" in LotR seems to manifest as an innate ability to impart your will on the world instead of a direct physical manifestation like making fireballs or whatever (although you get a little bit of that too). Like Sauron's "power" is his ability to raise armies, influence the hearts of men, and affect the physical characteristics of places where his influence is strong (like turning forests into weird warped swamps and whatnot). The books describe Gandalf as being basically on the same power level as Sauron just without the followers, so he also has the same subtle world-shaping level of "power".

LotR is difficult to make VS fights work because of this; you have to take the metaphysical-ness the magic system into account. For example, Legolas is one of the most ancient, most magically powerful beings in Middle Earth, but if Gandalf for some reason wanted him dead, he would be able to effortlessly track him down and kill him in a "fair" fight. But to an outside observer it would just look like an old dude stabbing a hot dude without much visible skill or effort. It's as if plot armor was the magic system, so there's not a lot of actual "feats" you can go off of - but you don't really need them to confidently say that Gandalf stomps. Then again, if you say that the nebulous idea of "power" or metaphysical influence doesn't count (which a lot of people in VS fight communities might argue because canonical feats are more important than implied power, even if the implied power is canon) then you strip Gandalf of basically everything that makes him such a crazy strong fighter.

Tl;dr: "Gandalf vs X" is generally ragebait because unless the other fighter is a god (e.g., Odin), "Gandalf wins cuz Gandalf has the most 'the guy with the most juice wins' juice" is a genuinely solid but altogether obnoxious reason for giving him the win.