>>39464604>>39464959It's not entirely clear how Gigantamaxing will work, but it's heavily implied on the site that only a "Special" member of the species can do it and you can get said special members from raids.
We know nothing about whether or not it's transferable, nor if there's a way to make your own Pokemon "special". There is something called "Dynamax Level" that they haven't explained yet that could be tied to it, but we simply don't know right now.
That said, Gigantamaxing really just gives the mon a new shape and a special move to replace one of the base Max Moves. It's not exactly necessary.