You can add your bank account to your Gemini account by:
Going to the Payment methods page and under Automatically link bank click on Get Started
Continue through the flow to enter your bank login credentials via Plaid
If you are unable to link your bank account via Plaid or prefer not using your online banking credentials to register your bank account, please exit the screen and navigate to Manually link bank and input your banking information. In order to validate your bank account and enable ACH transfers (Domestic users only) and withdrawals to this bank account, a successful wire deposit into your account will be required. For wire deposit instructions, please take a look at: How do I send a Wire Transfer?