How to get Gigantamax Urshifu
When you first access Kubfu in the Isle of Armor DLC, your goal here is to level him up and make him happy, which leads to his evolution to Urshifu. You can find more details on how to go through this process in this guide here first.
After conquering one of the two towers and transforming Kufbu into Urshifu, you must return to the Dojo. Once you're there, you'll meet a familiar face in Hop, who's there to learn more about Dynamaxing and Gigantamaxing while you're on Isle of Armor.
While you're here, Dojo Master Mustard will tell you that your Urshifu is Gigantamax capable, but the problem is, they don't like the taste of Max Soup per se. Max Soup is how you can give a Pokémon Gigantamax abilities by collecting Max Mushrooms around the Isle of Armor. Fortunately, there is something that can make Urshifu eat the max soup that you have to seek out to make it sweet.
The mustard then sends you and descend after this sweet nectar which will make the max soup sweet enough for Urshifu, starting with the concentration forest. Shortly after entering you will notice a stray Petilil that is lost. Hop tells you to go into the forest and find a Lilligant looking for him, so go and head through the forest. You'll have to look around a bit, but you should find the Lilligant before too long in the open. You need to get Lilligant back to where Hop and the Petilil are, but don't walk too fast or they'll get stuck. You will be given some type of nectar, but Hop says it's not quite it and you have to keep looking.
Now the next step is to find an Applin in the forest, so go find it. It should be located fairly close to where you found Lilligant before. You'll have to run after Applin a few times until he climbs a tree. Go to the tree and the Applin will land on you, leaving its nectar. Hop will try again and say it's wrong, letting you try something else.
Hop will then mention that a Vespiqueen is what you need, so head to the new marker on the map, which is Honeycalm Island. This is the very oddly shaped island in the northwest area of the Isle of Armor, so go ahead. Head to the center and you'll meet Hop again, where you'll then interact with a tree. Shaking this tree will suck you into a Pokemon Den and have to fight a Dynamaxed Vespiqueen. This fight is incredibly easy, as you can technically use your entire party. Just use what you need to take out Vespiqueen and you'll be brought back outside the tree with a comb of Max Honey in your possession.
Once you have the Max Honey, return to the Dojo and enter. At this point, Mustard will challenge you to a full Pokémon battle in the back, but that's not what we want to do first here. Instead, head to the left side of the Dojo in the kitchen and talk to the person on the stove. Give him your Max Mushrooms and Max Honey to get the Special Max Soup and your Urshifu will now be able to Gigantamax.