Doing transactions on xDai network will require xDai for gas. xDai is a stable coin pegged to the USD dollar. Do not confuse with the STAKE Token which is the governance token for the xDai network.
There are several resources available to acquire xDai :
Bridge DAI to/from xDai: https://bridge.xdaichain.com/
xDai have created some documentation here: https://www.xdaichain.com/for-users/bridges/converting-xdai-via-bridge/moving-dai-to-xdai
Exchange USDT to/from xDai (via BitMax): https://bitmax.io/en/basic/cashtrade-spottrading/usdt/xdai
Buy xDai with fiat on the xDai network: https://www.xdaichain.com/for-users/get-xdai-tokens/buying-xdai-with-fiat
Use the faucet to receive 0.01 xDai daily: https://blockscout.com/poa/xdai/faucet
Trade on BaoSwap (xDai chain) (https://alpha.baoswap.xyz/#/swap)