Dynamax is when a Pokemon gets HUGE and it's moves get more powerful, depicting the moves base power. The Pokemon's moves also get a secondary effect, such as Max Starfall, the Fairy Type Max move, where Misty Terrain is set up after the move is set up, or Max Ooze, the Poison type Max move, where the user and it's allies get a Special Attack Boost. Their HP doubles as well after Dynamaxing. However, to balance it a bit more, this only lasts for three turns. Gigantamaxing is the same thing, except the Pokemon takes on a new look and gets a new move for one type. This can be either a buff or a nerf to a Pokemon. Say, for example, Gigantamax Charizard. When it Dynamaxes, it will turn into a G-max Pokemon if you caught is as a Gigantamax in a raid den, or if you evolve the Charmander that you can get after beating the game. (I think) It's fire type moves, instead of becoming Max Flare, become G-Max Wildfire. This is a case where it's not as good. Max Flare, after usage, sets of Sunny Day, meanwhile Max Flare basically activates something similar to fire spin, except I think you can still switch out.
In Short, Gigantamaxing is almost identical to Dynamaxing, just with a different G-Max move instead of a Max Move.
It's Bootleg Mega Evolving!