Gas is the network fee Ethereum charges to process a transaction. It goes to the network, not to ENS.
Any onchain change costs gas: registering a name, renewing it, transferring it, or saving changes to its records. Signing in with your wallet is free.
There is no fixed price. The fee rises and falls with network demand, so your wallet shows the exact fee before you approve each transaction.
When you register or renew in the ENS App, gas is paid in the same stablecoin as the name. For anything else, keep a little ETH in your wallet for network fees (gas).
If a transaction fails onchain, you still pay the network fee.