I'd give Gaethje the most chance to beat Khabib because of his defensive wrestling and leg kicks but that's only in a world where Khabib gets too cocky and start showboating or something.
but If Khabib stays focused, good luck throwing leg kicks or use defensive wrestling.
I think strikes matter on who you train with, preferably similar skill, size, and built as your opponent to get used to the range and all that jazz but when it comes to grappling, training with a heavier olympic level wrestlers is going to be super helpful and unless you're stronger, heavier, and more skillful than Khabib's training partners, good luck with that defensive wrestling.
maybe best bet for Gaethje is mix everything up early to make Khabib be concerned with all the arsenal Gaethje possesses and trick him, something like what Khabib did to Conor, had Conor worried about the TD so much that Conor dropped his guard and was caught with that over hand right or what Werdum did to Hunt, have him worry about TD and when he reacted, a knee to the face.
Gaethje could mix up leg kicks and attempt TDs early and when Khabib is not expecting, hard shot to the dome.
but more than anything, Tony Ferguson should be fighting Khabib, not DP or Gaethje, UFC (dana?) are freakin assholes to strip him of interim belt for injuries happened during stupid UFC media obligations Tony was trying to fulfill, it's not like Tony missed weight or some shit like that, Fuck them.