Note: The default currency options available to you may be restricted based on your country of residence. 

You can change your default currency between any of the fiat currencies available in your country of residence. On desktop, look at the navigation bar at the top of the screen and click on the icon with a fiat currency on it (i.e., GBP, USD, EUR, COP, etc.). When you click on that icon, you’ll be presented with a dropdown list of other available currencies.  Select any other currency from this drop-down menu to change your default currency. 

You can also change your default currency on the Payment Methods Page by selecting "Change" under "Default Currency," then select another currency from the pop-up menu.
On the Gemini App, you can change your default currency by navigating to the account tab, and then tapping on the currency under “Default Currency.”