Gems are needed used to upgrade your troops. Do not spend Gems to upgrade troops, such as Grenadiers, that are used infrequently. You can randomly get Gems as a random reward by completing the Boss levels in every single player campaign mission. You can play each of them five times per day to farm gems. Gems may also be picked up as a daily log in reward, free spin award, and Treasure Box item. They also may appear after leveling up or completing certain missions. The easiest way to obtain Gems is to complete single player campaign missions with a three star rank in all stages.