Currently, on the testnet, the fee token is ETH because that's the native token for Optimism. Our plans are to support the OMG token as the fee token on OMGX. Ideally, we'd like to do both natively, but supporting two native tokens is going to require non-trivial engineering work. So we're prioritizing supporting the OMG token as the fee token initially. The longer-term vision is to make it as easy as possible for OMGX to support multiple fee tokens or the popular ones.