LINK tokens are used to pay node operators for retrieving data for smart contracts and also for deposits placed by node operators as required by contract creators. The smallest denomination of LINK is called a Juel, and 1,000,000,000,000,000,000 (1e18) Juels are equal to 1 LINK. This is similar to Wei, which is the smallest denomination of ETH.
The LINK token is an ERC677 token that inherits functionality from the ERC20 token standard and allows token transfers to contain a data payload. Read more about the ERC677 transferAndCall token standard.
xDAI is used to pay for transactions on Gnosis Chain mainnet. Use the xDai Bridge to send DAI from Ethereum Mainnet to Gnosis Chain and convert it to xDAI. Use OmniBridge to send LINK from Ethereum Mainnet to Gnosis Chain.
Testnet ETH is used to pay for transactions on Optimism. Use the Optimism Bridge to transfer testnet ETH and LINK from Ethereum Goerli to Optimistim Goerli. Select Optimism Goerli in your wallet to access the Optimism Goerli bridge. Testnet LINK is available at faucets.chain.link. Testnet ETH is available at goerlifaucet.com or the faucets listed at faucetlink.to/goerli.