How do I switch off one payment gateway for a while without losing its keys?

Go to Settings → Store Settings → Payment Gateway and find the gateway in the Payment Methods table. Untick its box in the Enabled column. It stops showing at checkout straight away.

Or click Configure, set Status to Inactive and click Submit.

Your keys stay saved either way. When you want it back, tick Enabled again (or set Status to Active). The Live at checkout badge on the row shows which gateways customers can currently use.