1 ETH = 1e9 gwei = 1e18 wei. Explorers and gas trackers speak gwei because whole-ETH gas prices would be tiny decimals.
If an oracle shows 20 gwei and your transfer uses 21,000 gas, the base fee estimate is 20 × 21,000 gwei, which you can convert to ETH with a unit converter.
Priority fees (tips) are also often expressed in gwei. Your wallet may show max fee and priority fee separately.