ERC20 is a set of standard requirements and functionality for tokens built on the Ethereum network. ERC20 tokens inherit all the benefits of the Ethereum network without having to move between blockchains.
Supported ERC20 tokens are deposited to Ethereum addresses on Gemini. ERC20 tokens can also be withdrawn from Gemini to Ethereum addresses. It is important to make sure that the receiving Ethereum address has support for the token you send it.
You can find a list of supported ERC20 tokens on Gemini here.