From December 4th, you will be able to catch Gigantamax Snorlax Pokémon Sword and Shield. Gigantamax Snorlax will be added to the Rare Encounter Schedule of Max Raid Battles, giving you the option to catch him whenever you take part in a Max Raid Battle. These potential encounters will last until early January, so you have about a month to catch him.
Max Raid battles involve a team of Trainers facing a very powerful Pokemon. You can join them as soon as you arrive in the Wild Area, but they are challenging at that stage of the game. Look for glowing red rock formations with a beam of light coming from them to join a Max Raid Battle. While it is possible to take part in these battles with AI teammates if you are offline, you will be able to get Gigantamax Snorlax only by taking part in online battles.
When facing Gigantamax Snorlax, you'll need to watch out for his special G-Max Replenish move. This powerful move deals damage and restores berries Snorlax and his allies ate during battle. When you defeat the Snorlax in a Max Raid battle, you will have the opportunity to capture it.
While there is no guarantee that any Max Raid Battle you take part in will feature a Snorlax Gigantamax, there will be a greater chance of encountering it on other types of Pokémon during the event. There are also methods that allow you to obtain Gigantamax Meowth, Eevee and Pikachu under particular conditions. Meowth is available as an early purchase bonus, while Eevee and Pikachu are only available to people with a relevant Let's Go save file on their Switch.