There is no BTG blockchain yet -- at least not a public blockchain. The code is not public yet. The only thing that has happened is the snapshot of the UTXO set. According to their statement, they will credit users BTG and make it available for withdrawal (assuming it launches at all). They made no promises to allow trading.

I don't understand how Bitfinex is allowing trading right now. Maybe they are in communication with the developers and have confirmation that the blockchain has been launched. It seems irresponsible to let people trade when the coins may not even exist yet.