I think Bittrex will only provide support for Bitcoin Gold by crediting all Bitcoin account balances on their exchange. However, they will not support the immediate trading of BTG, so their clients still need to wait until it is live or tradable. Or, if their clients can't wait, they can just transfer it to another exchange with support for it, e.g. YoBit or HitBTC, and sell it for BTC or any other coins of their choice.