All of which were part of the rebellion against Illuvatar. Assuming Gandalf doesn’t rebel (which seems a pretty safe bet), he wins.
It may take time, his body may be destroyed once or twice, but he’s a freaking Maia, a servant of the being that created… well… everything, and Illuvatar doesn’t do ‘losing’.
I think it depends on what kind of “fight” you’re talking about. If it’s hand-to-hand (or lightsaber to elven sword) combat, Vader has much more energy and weaponry. Vader is a warrior, and a servant. But if it’s a long-haul strategy, Gandalf inspires all the little dudes to help him (note how even Ewoks can defeat most Imperial weaponry) and outwits Vader. Gandalf isn’t a warrior, he’s a captain and strategist.
I think Gandalf would don the ring and kick Vader’s ass.
Then there would really be a problem.
Guys, we don’t even know if Vader can run. Gandalf DoTs him to death.
I’m not sure how much use the Force can be against someone who participated in the Song of Creation and presumably has the Force running through every strand of his being.
I thought that Gandalf’s order was not fully explained, even in the Silmarillion. Probably a Maia, maybe not a very strong one, IIRC.
On the other hand, being able to beat a balrog in a fair fight puts him near the top of any sub-Valar scale.
I think he would just pull out his alter-ego Magneto powers, and win in a walk. 
As I understand it, the Maiar are specifically forbidden from manifesting their full power in the affairs of Middle Earth - they’re meant to advise mortals, not fight their battles for them. So we only see a barest hint of how powerful Gandalf really is.
Vader, however, is not from Middle Earth. As an invader from an alternate narrative, his meddling would be just as destructive to Iluvatar’s plan for Middle Earth as an unfettered Maiar running around. So I can see him saying to Gandalf, “For this guy? Go all out.” And Vader ends up as a pillar of salt, or something.
However, if the smackdown occurs in a galaxy far, far away, Gandalf may have nothing to draw upon.
If it happens here, neither may have special powers. Vader has better tech. If Gandalf can manage to outlast Vader’s batteries, he would have the opportunity to exercise his vaunted mercy. Unfortunately, for Gandalf, I’m guessing that since in his childhood Vader built robots and pod-racers as after-slave projects, he could probably tool up a serviciable power source even using our mundane tech.
BTW, what does “Vader” even mean? Does it even mean anything?
That may work both ways, though. If Iluvatar has no power in the Star Wars 'verse, it seems logical that the Force would have no power in Middle Earth.
I’m generally pretty good at figuring abbreviations/acronyms out but “TK” has stumped me since I first saw it in a HULK magazine 20-odd years ago. Can someone help me out?
It’s short for “telekinesis.”
Gandalf is explicitly stated to be a Maia. At one point in the trilogy, he says “Olorin I was called in my youth in the West”, and the Silmarillion lists Olorin as the wisest of all the Maiar. There’s also more detailed info in Unfinished Tales, though one can debate how canonical that is.
nemmind
It seems to me the video has some of the dialog incorrect.
After Gandalf proclaims “You shall not pass”, no matter water Darth replies, his next line should be “What is your favorite color”.
And after Gandalf slices off Vader’s hand the dialog is supposed to be…
Gandalf: Now stand aside, worthy adversary.
Vader: 'Tis but a scratch.
Gandalf: A scratch? Your arm’s off.
Vader: No it isn’t.
Gandalf: What’s that, then?
Vader: [after a pause] I’ve had worse.
Gandalf: You liar.
Vader: Come on ya pansy.
German for ‘father’
Dutch.
Vadar would win in a direct confrontation but Gandalf would avoid this and if he could not avoid it he would come back. In both cases he would guide those needed to take down Vadar and the Empire.
Gandalf overthrows the Empire?
I guess that explains the Ewoks.