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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.
Mega Evolution is unavailable in Pokémon Sword and Shield, due to the lack of Key Stones and Mega Stones.
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Gigantamax (Japanese: キョダイマックス Kyodaimax) is a special kind of Dynamax introduced in Generation VIII.
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The eighth generation (Japanese: 第八世代 eighth generation) of Pokémon games is the eighth installment of the Pokémon video game series. The eighth generation began with Pokémon Sword and Shield
sourceTherefore, it’s not possible to mega evolve and gigantamax any Pokemon in a game.