On the Ethereum blockchain, gas refers to the cost necessary to perform a transaction on the network. Avalon blockchain doesn't have any fees for transactions. However, when you are swapping Dtube Coin to Wrapped Dtube Coin(WDTC) you need to interact with Ethereum blockchain and need to pay the gas fee. Because of that there will be a fee paid in Dtube coin which is equivalent to the price of 1 transaction on Ethereum Blockchain.