Alright.
For starters, I'm going to assume Gargos's reality-warping is restricted to his dimension, since A) that's what it says on his profile and B) I haven't been shown any real evidence that contradicts that.
Anywho, Gargos's minions? They'll make great distractions, but they're going to die the moment they step to Doom. Like I said, he one-shots monsters who take hits from a gun that's almost three times more powerful than the feat the KI cast scale to, and I'm pretty sure Gargos's minions are weaker than the cast of KI. I'd need some info on Gargos's juggernauts, but it's also already been proven in-game that being bigger than Doom doesn't stop him from ripping you apart. Doom also has plenty of experience fighting enemies who summon minions (in the form of both the Arch-Vile and the Summoner), so he'll know enough to attack the source as well.
I won't ignore Gargos's advantages, though. That bastard can fly, and he's bringing good regen into the fight. Portal attacks put his ranged game well above Doom's, and he has more ways of actually harming Doom than vice-versa. (Doom only has melee attacks, the BFG and maybe the Gauss Cannon) I also don't know about an answer to the illusions.
I would actually argue that mind manipulation isn't that big of a factor. Despite Doom not having 'resistance to Mind Manipulation' on his profile, he completely (and regularly) ignores the negative side-effects of Hell's powerups, which include driving the user out of their mind. I feel like that should give him some sort of mental resistance that would help him deal with whatever Gargos's mind manip is.
Portal attacks would most likely be rendered useless if Doom managed to get in close, and it's completely within the latter's character to do that.
They've both got countless years of combat experience, so I won't argue about tactics here. Both of them also have experience fighting opponents similar to each other; Gargos has fought gun/energy weapon users in the form of Fulgore, and Doom (in addition to the thing about minion-summoning foes I mentioned) has fought beings that can fly in the form of the Lost Soul, the Revenant, the Summoner and the Cacodemon.
Super modes? Gargos's Instinct Mode is actually a bad thing. Heavy attacks from guys far weaker than Doom have punched through that armor, and Gargos reducing his mobility is basically him begging to take a BFG blast to the face. Meanwhile, if he's allowed to have it, the Berserker would give Doom an immense advantage in this, since it puts his physicals even further above Gargos's to a ridiculous degree. If Doom gets Quad Damage, then that's another really huge swing in his favor, since a data log in DOOM 2016's campaign actually describes the powerup as being a literal 4x boost to any weapon. (A guy who used it managed to decapitate three people with just a pocket knife.)
All in all, my only real concern here is whether or not an IC Gargos will just BFR Doom during the fight, since I don't think Doom has anything that'll bring him back into the battlefield if he gets flung out of it. If that's in Gargos's character, then my vote goes to him. If not though, then I think Doom takes it after a long and brutal fight. It's a battle of versatility VS sheer might, but in this particular case, Doom's power and experience would be enough to run through most of Gargos's options. Doom also loves aiming for the head when he kills things, so that would be enough to get past Gargos's Low-Mid regen.
(P.S.: I would pay good money to see this fight happen. The matchup is really good.)
