I am delighted to report that I have never seen an "Ultrakill" and therefore am unable to adequately understand your doubtless hilarious/eye-opening/(insert adjective here) reference.
Ultrakill is a video game which could be summarized to: Doom but fast.
For some reason, its community is very queer and very horny.
An artist drawing porn of Gabriel was hired by the developers to create textures.
Another developer released an official Buttplug.io Mod for rumble support
Although only tangentially related, the voice actor for Gabriel, Gianni Matragano, has branched out. Into a FNAF furry porn game.
Ah, sadly it's a singleplayer game. There are multiplayer mods (I think?) but I haven't tried any of them and I'm not sure how great of a first experience they are. It's a solid singleplayer game though with an extremely high skill ceiling.