There are several Pokémon that you need to encounter in Pokémon Brilliant Diamond and Shining Pearl that you need to find to complete your PokéDex. Some of them may have evolution requirements that you need to meet to catch their evolved forms. Here’s what you need to do if you want to evolve Gligar into Gliscor in Pokémon Brilliant Diamond and Shining Pearl.
Gligar needs to be holding a specific evolution item before you can evolve it. You need to find Razor Fang, one of the harder evolution items you can find in Pokémon Brilliant Diamond and Shining Pearl. The reason it’s difficult is that you can only find this item at the end of the game, right before the Battle Tower, in the garden. You can also grab it inside the Battle Tower using Battle Points, so you’ll need to participate in the battles if you want to evolve Gligar.
Once you have Razor Fang, make sure Gligar is holding it, and then level it up. You also want to make sure you’re doing this at night. If it’s not dark, Gligar won’t evolve when it levels up, and you’ll need to wait for the next level for this to happen.