To use the network "Gnosis Chain" with MetaMask, you must manually add Gnosis Chain to your MetaMask. The first step is to select the "Custom RPC" option in the list of your networks in MetaMask.
Then, you need to paste these details:
Network Name: Gnosis Chain
New RPC URL: https://rpc.gnosischain.com
Chain ID: 100
Block Explorer URL: https://blockscout.com/xdai/mainnet
You click "save" and Gnosis Chain is now added to your MetaMask!