Yobit is already running a BTG/BTC futures trading since last night.
HitBTC will start it within 24 hours from now.
Bittrex will credit its user BTC with an equivalent amount of BTG, but didn't promise to provide a new market for BTG. But they're likely to create a new market for BTG once BTG has a fully supported wallet
Bittrex will make all reasonable attempts to credit our users BTG but until we have a fully supported wallet, we can neither guarantee tokens or consider creating a market. We will continue to monitor the state of the codebase and network. We'd also like to encourage the BTG development team to reach out to us.
https://support.bittrex.com/hc/en-us/articles/115002320451-Statement-on-Bitcoin-Gold-BTG-