Gigantamax is a Generation VIII transformation in which a Pokémon with the Gigantamax Factor grows to giant size for three turns and also changes appearance, gaining a unique G-Max move. Only 32 species can Gigantamax, across 33 different Gigantamax forms.
Dynamaxing in Pokémon Sword and Shield works on any Pokémon: three turns at giant size, doubled HP, and moves converted into Max Moves. Gigantamax is the rarer version of the same system. The Pokémon needs both the right species and the Gigantamax Factor, shown by a red symbol on its summary screen. A Charizard without the factor only Dynamaxes into a bigger Charizard. A Charizard with the factor becomes Gigantamax Charizard and replaces its Fire Max Move with G-Max Wildfire, which keeps damaging non Fire opponents for four turns.
Bulbapedia counts 32 species capable of Gigantamaxing and 33 Gigantamax forms, because Urshifu has a separate form for Single Strike and Rapid Strike, while Flapple and Appletun share one visually identical form. Each G-Max move carries a side effect normal Max Moves do not, such as Gigantamax Lapras setting Aurora Veil with G-Max Resonance or Gigantamax Snorlax planting berries with G-Max Replenish.
Most Gigantamax Pokémon came from special Max Raid Battles, and the Isle of Armor expansion added Max Soup, which grants the Gigantamax Factor to a compatible Pokémon. Like Dynamax, the effect lasts three turns, so wasting one turn on a switch or a status move is a real cost.
In play"That raid Charizard can Gigantamax, so keep it and train it up."