I want to preface this by saying that I don't like the "Justin 2.0" moniker for a number of reasons, but I'll use it just because it's what a lot of people recognize. This fight is interesting because depending on the "versions" of each that you use, they counter each other very effectively in either direction. I'll try to illustrate what I mean.
If this was Justin prior to the Dustin (or Eddie) bout, then I'd favor Nate and pretty heavily at that. Justin would go out there and try to finish Diaz on the feet early, especially if Nate got in his head with a slap, trash talk, flipping him the bird, etc. Justin would land some hard shots, with the most significant ones being his leg kicks. Nate would outbox him steadily, though he'd probably lose the first round. Justin would ultimately gas out trying to knock Nate's head off, whiffing on a lot of his shots with Diaz pulling way ahead on the total/significant strike counters and his gas tank looking to be in much better shape. Unless he manages a doctor stoppage or leg kick TKO, Justin ends up getting TKO'd or submitted late in the fight (or losing a lopsided Decision like Johnson did). Nate is a bad stylistic match-up for punch-drunk brawler Gaethje. I'd favor Nate at ~70/30 on this, give or take. Not impossible for Justin to win simply because of his leg kicks, but certainly not looking great for him IMO.
Using "Justin 2.0" with his head screwed on relatively straight and Trevor in his ear, I think he takes it more often than not. He showed five-round cardio against Tony so long as he's able to keep the fight on the feet and land clean shots. His boxing is improved (though you wouldn't know it from the haymakers he was winging at Khabib). Nate is still the better boxer of the two, but I feel the gap is sufficiently narrowed that we would see a dynamic like Holloway vs. Poirier, where even when Max was outlanding Dustin, the latter's strikes clearly had more oomph behind them which clearly resounded with the judges. And again, the leg kicks. Nate even in his prime never fully "solved" the leg kick dynamic, while Justin for all his flaws is acknowledged to be one of the most dangerous leg-kickers in the sport today. That's a bad combination.
I don't think it's at all unreasonable to believe that with his slightly improved boxing, more conservative approach toward his gas tank, and signature leg kicks that Gaethje would be able to stay ahead on the scorecards of each round by simply by landing the more impactful shots. So long as he doesn't gas, the main threat would be a Hail Mary sub -- he's defended himself against submission specialists taking his back in WSOF, but none of those were Nate. However, since the Diaz bros don't wrestle, the sub attempt would have to come during some kind of scramble and Justin has shown himself to have pretty good kinesthetic awareness and speed during those types of situations. I think he wins a comfortable Decision, maybe dropping a round as his output slacks off late in the fight or if he makes some kind of mistake. Failing that, maybe he wins a late TKO/doctor's stoppage.