Every address that has a balance in BTC before the fork will receive an equal amount of the (forked coin) in this case BTG. The BTC private key has the same amount in different units on a different chain. To redeem the forked coin you should import your private key to a supported BTG wallet. From there you can claim your BTG and make a transaction to an exchange address. But keep in mind that Coinbase doesn't show your private key, so you won't be able the forked coin till Coinbase decides to support this chain.