The rewards from staking are not fixed and are determined by the relevant protocol. You will receive rewards in the amount determined by the relevant protocol (such as Polygon or Ethereum) minus a 15% fee that Gemini retains for paying on-chain fees and acting as the staking service provider. 

Information about Ethereum staking published by ethereum.org, including total ETH staked, total validators and current APR, is available here. ethereum.org is a public, open-source resource for the Ethereum community that anyone can contribute to.

Information about MATIC staking published by polygon, including total MATIC staked, total validators, total rewards distributed, and commission rates for each validator, is available here. Gemini uses Figment as a validator for MATIC.

You can learn more about staking rewards in the FAQ page for ‘Why are staking rewards not fixed?’