The Ethereum network charges ‘gas’, small denominations of its native currency ETH, to process transactions. In your Contract Wallet, the Gas Tank is where you store the ETH needed to pay gas to process transactions. To top it up, hit the fuel gauge icon underneath your Contract Wallet balance on the ‘Wallet’ tab. You must not empty your Gas Tank completely; gas is required to pay for every transaction, including topping up your Gas Tank. It’s therefore a good idea to send some ETH to your Contract Wallet from a separate address once you’re onboarded.