For an automatic process, go to Chainlist, search for Gnosis, and connect your wallet.
For a manual process:
Follow the steps found here to add a custom RPC network.
Then follow the steps found here for the custom network settings; specifically:
Network Name: Gnosis Chain
New RPC URL: https://rpc.gnosischain.com
Chain ID: 0x64 (the hexadecimal equivalent of 100; if you encounter issues, input 100 instead and retry)
Block Explorer URL: https://blockscout.com/xdai/mainnet/