The Ethereum network charges an ETH fee, called gas, to process transactions on its blockchain. If you have an Ethereum-based wallet, you must maintain enough ETH to cover gas fees. BitGo stores this ETH in a gas tank - a separate reserve from the rest of your wallet balance.
Because new wallets must broadcast their address on the blockchain, you must fund your gas tank during wallet creation. If you have insufficient gas in your gas tank, you can't transact on the Ethereum network.
BitGo wallets for ETH and all tokens on the Ethereum network require a gas tank. To determine if a token is on the Ethereum network and needs a gas tank, see Coins and Tokens.