“It is capable of Gigantamaxing under the right conditions, such as during Max Raid Battles in the Pokémon Sword and Pokémon Shield games. You will not be able to feed Melmetal Max Soup, which is available in The Isle of Armor, the first round of content for the Pokémon Sword Expansion Pass and Pokémon Shield Expansion Pass. (Note: Full version of game required to use Expansion Pass for that game. Games and Expansion Passes sold separately or in special bundle packs.)”
So, yeah, no plans to add G-Max Melmetal outside of events. Also gotta love "you can't use the expansion pass for this. Buy our expansion pass!"