There's a secret shop in Gerudo Town, selling some of the game's most interesting clothes, including the Radiant clothing set. The store is well hidden in The Legend of Zelda: Breath of the Wild's Gerudo Town – you'll need to be observant to unlock it.
You'll need to unlock Gerudo Town first, which means getting your hands on the right clothes (this is a major quest I don't want to spoil, but search the Kara Kara Bazaar for clues).
When you enter the town from the entrance closest to the shrine out the front, look at the second walkway to your left. There's a door down here leading into a secret store, but to enter you'll need the password. Don't worry about that right now, because you don't have it, and when you guess wrong you'll have to wait a day before you can try again.
Instead, head to the milk bar on the other side of town. Go through the door, and then into the outside area where several Gerudo are having a chat.
These women here have the password, but they won't give it up easily.
They'll tell you to go away, but you'll notice that there's a grate behind them...
Go into the house next door. From here, you'll be able to get the password.
Go up to the grate again and listen. The women will wonder what your deal is, exactly, but they'll also share the password...
"GSC" then a diamond. GSC stands for "Gerudo Secret Club," which is easy enough to remember.
Go back to the other side of town, and down the alley you noted earlier. Give the password at the door and you'll be allowed in.
Side quest completed! And now, you can look at the secret club's wares.
The Desert Voe collection is a stronger alternative to the outfit you're already wearing in Gerudo Town. It's a nice bonus for completionists who want a stronger heat-resistant outfit.
The Radiant Set is perhaps more exciting, if only because it glows. The actual advantages of the suit aren't amazing – it lures skeleton enemies and makes bone weapons more powerful – but it's one of the best looking sets in the game. To make it, however, the shopkeep needs you to bring her Luminous Stones. If you don't have any, there's an easy way to gather them.
You just need to head to the Sho Dantu Shrine, which we have already written up a guide for. The shrine is hidden, but it's also surrounded by Luminous Stones, which respawn after a while. Once you have this shrine marked as a warp point, you can head there throughout the game to gather them. You need nine in total for the full Radiant set, and more to upgrade them.