There are several Pokémon that you must encounter in Pokémon Brilliant Diamond and Brilliant Pearl that you must find to complete your PokéDex. Some of them may have evolutionary requirements that you must meet in order to capture their evolved forms. Here's what you need to do if you want to evolve Gligar into Gliscor into Pokémon Brilliant Diamond and Brilliant Pearl.
Gligar must hold a specific evolution item in hand before you can evolve it. You need to find Razor Fang, one of the hardest evolutionary items you can find in Pokémon Brilliant Diamond and Brilliant 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 get it inside the Battle Tower using Battle Points, so you'll need to participate in battles if you want to evolve Gligar.
Once you have Razor Fang, make sure Gligar is holding him, then level up. You also want to make sure you do this at night. If it's not dark, Gligar won't evolve when he levels up and you'll have to wait for the next level for that to happen.