A linear perpetual is traded and settled in GUSD for non EU & EEA users, or USDC for EU & EEA users, and not the underlying cryptocurrency, such as BTC. As for the BTC linear perpetual, if the price of BTC in GUSD/USDC goes up, the value of the contract goes up and the P&L is generated in GUSD/USDC. Similarly, if the price of BTC in GUSD/USDC goes down, then there is a negative P&L in GUSD/USDC. 

Please see What are the eligibility requirements for  trading perpetuals? for further information and for a complete list of the countries in which Gemini Derivatives are available.