Gems are in-game currency that can be used for various purchases including Card Packs, Decoys, Power Packs and Energy Packs. Gems can also be used to purchase various promotional items from the in-game store, such as the Starter Kit, which is limited to 1 per account.
Gems can only be obtained via purchase with real money. They can bought at the in-game store in various bundles. Players purchase gems via their Apple ID, so iTunes credit can be applied towards the purchase of gems.