Funds can be bridged from Gnosis Chain (formerly known as xDai) to Optimism or any other network from the app's homepage.
Click on "Deposit" for any of the supported collateral types:
Select "Bridge USDC" or "Bridge ETH".
Open the dropdown menu underneath the "From xDai" option. A list of bridges will be shown (Hop Exchange, xPollinate and O3 Swap).
Some useful security tips for bridging: i) before bridging, check the bridge status on Twitter (search the bridge name and see if there is any chatter about an exploit or a hack). If there's no chatter, then it should be safe to bridge. ii) Set the approval to the exact amount you're bridging so that even if there's an exploit, your funds will remain safe. iii) Always DYOR before using any bridge.
Select one of the options and a new window will be opened in your browser. For example, once clicking on Hop Exchange, you'll see the following screen:
Connect your wallet and set the approval to allow the bridge to use your ETH/USDC.
Once you have approved Hop Exchange to use the ETH/USDC in your wallet, you can then enter an amount to send from Gnosis chain to Optimism.
Click "Send" and confirm the transaction in your wallet.
If you want to transfer funds to a network other than Optimism, you can change the destination chain as shown below: