How do I trade Dai for USD on Gemini ActiveTrader?
Why you want to trade stable coin to USD?
With GBP in your account on the Gemini Mobile App or on your desktop, you can buy cryptocurrencies like Bitcoin (BTC), Bitcoin Cash (BCH), Basic Attention Token (BAT), Dai (DAI), Ether (ETH), Litecoin (LTC),
ChainLink (LINK), Orchid (OXT), Amp (AMP), Pax Gold (PAXG), and Compound (COMP).
From this sentence it appears to me that you may try to get rid of GBP buying anything from the list above and than use them to trade on all trading pairs available on gemini?
If it didn't work out try different exchange - like binance. I have never heard anything about UK restrictions there.