We don't know if the Gambit in question will be the version from D&W. It seems rather unlikely, TBH, given that raises the question of where X-23, Blade, and Elekra are, let alone Wade and Logan. I'd guess he's more likely he's playing a Foxverse Gambit who we somehow never quite saw onscreen.
Admittedly, it's almost certainly the Beast from The Marvels. Though unless Monica Rambeau is also in Doomsday, what we find out in the post-credits scene will be pretty skippable.
The Fox X-Men movies were so loose with continuity they have lots of different options for what would be the "canon events" they choose to recognize here.
Admittedly, I forgot about that, since Danny Ramirez had absolutely zero impact on that story.
Similar to Walker, he got a feature film to act as his re-introduction.
Anyway, my point was I'm worried the flop of the Marvels (which was heavily predicated on understanding aspects of Wandavision and Ms. Marvel), may have scared Disney away from having characters/plot points which are meant to directly follow-up on Disney+ exclusive content, so the shows may be shoved into a semi "beta canon" position going forward, where the shows reference the movies (and may even have movie actors dip in), but the movies don't reference the shows anymore.