xDai is an EVM based Ethereum sidechain. Learn more about it here: https://www.xdaichain.com/
Please note: xDai and Ethereum mainnet are completely separete blockchains. A Safe on Ethereum mainnet does not automatically exist on xDai. DO NOT SEND MAINNET FUNDS TO AN xDAI ADDRESS, unless you know what you are doing. Also, dapps are different on both networks. Not all Ethereum mainnet dapps exist on xDai and vice versa.
The Gnosis Safe web interface is available for xDai at https://xdai.gnosis-safe.io/app/.
In order to interact with it, you need to connect with a wallet that supports xDai.
Metamask does not support xDai out of the box. This is how you configure xDai on Metamask (Inspired by the Metamask setup guide from xDai):
Network name: xDai
Chain ID: 100
Block explorer URL: https://blockscout.com/poa/xdai
Hit "save" and select xDai in the network dropdown. You should now be able to connect to https://xdai.gnosis-safe.io/app/ with your Metamask.
Metamasks gas estimates are sometimes a bit off on xDai. On xDai you should be able to use a gas price of 1 in almost all cases.
An alternative to Metmask on xDai is the Nifty Wallet from POA Network.