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 network. You can also head here and click 'Add to MetaMask' in the top-right corner to quickly add it.
If you need the custom network details to add it manually, use:
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/