On Avalanche, transaction fees are paid in the native asset, AVAX. In order to send transactions on the Avalanche C-Chain, you must have sufficient AVAX in your wallet to cover the cost of the gas for the transaction. To help you get started on Avalanche, the bridge will airdrop you a small amount of AVAX if you move more than $75 (subject to change) worth of tokens over from Bitcoin. In order to avoid running out of AVAX to cover your transaction fees, we recommend first obtaining an adequate amount of AVAX. You can acquire AVAX on Pangolin or any other exchange.
For a more in-depth read on how the Bitcoin Bridge works, please read the following articles.