Gems represent a resource that is used for unlocking Player Tiers and can be obtained in the Shop - Currencies section. To check your current Gems balance, make sure to open the player profile and click on the button ‘Tier up’ in the OVR info button. The balance will then be displayed in the upper right corner. 

You can also exchange eligible players from your squad to reduce the amount of Gems needed for the next tier-up of a player. Exchanging a player removes him from your squad permanently. This action is irreversible, so make sure to plan accordingly.