Gigantamaxing, and Gigantamax Pokémon, are a new feature in Sword and Shield that change the form of certain Pokémon when Dynamaxing in battle.
There are only a few Gigantamax Pokémon available in the series and they are honestly quite rare to come across.
The only way to find Gigantamax Pokémon of any kind is through Max Raid Battles in the Wild Area. You will need to search each glowing Raid Den to see if it houses a Max Raid Battle and, if it does, take part in it.
Even then the chances of capturing the Gigantamax Pokémon aren’t guaranteed as you will need to first beat it in the raid with fellow trainers or NPC characters and then hope that you are successful in capturing it afterward.
If you fail, you will need to scour the Wild Area once again for another Max Raid Battle, fight it, and try to capture it all over again. So be sure to have an Ultra Ball on hand when going into battles with Gigantamax Pokémon to give yourself the best chance possible.
Of course, there are other ways to get Gigantamax, with Pikachu, Eevee, and Meowth all having a Gigantamax available through Events and Mystery Gifts, so we expect something like this to also happen in the future with different Pokémon.