Pokemon Sword and Shield: Can I change my uniform when battling a gym?

Though Pokemon Sword and Shield seems to imply that you can change your Gym uniform by giving you the ability to buy and obtain new uniforms, you just can't. There's no explanation for it. You're stuck wearing the same default uniform whenever you enter into a Gym challenge or the Pokemon League Championships throughout the game. That uniform is the same for everyone who plays the game (with a slight sizing variation for girl and boy trainers).

You can choose the number on the back of your jersey when you first register for the Pokemon League early in the game. You can also get haircuts and change your hairstyle, makeup, and eye color, all of which will remain the same when you enter a Gym match. Finally, while hats will disappear in Gyms, you do get to keep any eyewear you have on. Glasses wearers don't have to assume their character is unable to see their Pokemon when they start their battles.

This all begs the question: what the heck those Gym Leader uniforms you keep getting are for? You can wear them or buy the "types" that don't have a designated leader from a vendor in each Gym, but ironically you can only don them outside of Gym matches. Simply go to a clothing store in a town like Hammerlocke or Motostoke and use the closet. You'll see the items in your inventory.

The good thing about the Gym Leader uniforms is that you don't have to wear them in full. You can mix and match pieces just like any other piece of clothing. For instance, you can pair the ice-type leader's shirt with the Fairy-type gloves and a bunch of other non-Gym Leader clothes. It's a quick, free way to expand your wardrobe if that's a part of the game you're interested in.