Why is there a gas fee, and why does it keep changing?

Anything that writes to the blockchain — moving an NFT, sending tokens — costs a network fee. It goes to the network, not to Creta, and it is charged on top of the price of whatever you are buying.

The figure shown before you confirm is an estimate: fees move with how busy the network is, so the final amount can differ slightly. Keep a little of the network's fee token in your wallet — a transaction with nothing to pay the fee with simply will not go through.