A gem with its price
Gems are used for Powering-up Necklaces or Anklets. They can be bought in the shop under the Academy tab for 1000 GP each. They can also be used to feed pets.
Another use for them are for unlocking certain characters with their cash mission or other quests.
In Grand Chase Dimensional Chaser, Gems serve more prominently as a separate currency of higher value to purchase exclusive items and other currencies in-game such as Prana and Gold. Additionally, they also serve as payment for reentering Battle System modes, such as Daily Defense and Dimensional Boss. Gems in the game can be earned via microtransactions or events.