Some games do participate in reward systems like that, but as far as my knowledge goes, this is not one of those games. The limited gem rewards are through gameplay missions and quests, and watching ads through mystic vision.
Other off-game rewards are done through contests here, and through the official SG accounts on Facebook and Reddit. But these are usually small little things like the ♾ The Thanos Challenge – A Competition of Skill & Creativity, or something like “post the answer to this question and you have a chance to win!” or whatever
Anyone claiming they’ll give you gems if you download an app or whatever… is lying to you