If a payment gateway is removed how are new transactions processed?

If there are multiple payment gateways currently configured on your site then all new transactions will be routed to those other gateways for processing. For accounts with multiple gateways configured, Recurly will route transactions based on accepted card type and currency. Beyond those settings, Recurly will send the transaction to the gateway first added to the account. You are able to specify which gateway will be used per transaction by utilizing Recurly's Custom Gateway Routing functionality (https://docs.recurly.com/docs/custom-gateway-routing-configuration). We do not recommend completely removing a payment gateway for the fact that if you need to issue refunds for payments that are processed via that gateway you will need to have it on your account to do so. You can disable new transaction processing disable in the gateway configuration options for a few months in that case.