At launch you will need GUSD in your Gemini Derivatives account in order to trade perpetuals.
Collateral is measured as Margin Assets Value. You can see your Margin Assets Value in the Order Entry panel when placing your orders.
Gemini will soon make other assets available to contribute to Margin Assets Value. This feature (Cross Collateral) will allow customers to collateralize their assets to borrow against another asset. Stay tuned to our blog for the latest updates.