Mega Gengar cannot Gigantamax as Megas and Dynamax have not ever existed in the same games.
Though for something like NatDex on Showdown!... that’s actually kind of a good question. I would think it wouldn’t, even if they ever existed in the same game (but I doubt they will) as this would be too overpowered, plus the Dynamax button and the Mega button (and Z-Power too, but that’s not important here) are pretty much on the same spot.