This issue has been updated here ===> @zero21mill/statement-from-bitcoin-gold-to-coinbase
QUOTE --
We operate by the principle that our customers should benefit to the greatest extent possible from hard forks or other unexpected events. However, safety and security are also important considerations for any asset supported by Coinbase.
At this time, Coinbase cannot support Bitcoin Gold because its developers have not made the code available to the public for review. This is a major security risk.
Coinbase's current priority is to add support for Segwit2x, followed by Bitcoin Cash. We will continue to monitor the development of Bitcoin Gold, and if its network proves to be both secure and valuable, Coinbase may decide to support it at a later date.
Coinbase/GDAX will not sell or keep Bitcoin Gold associated with customer Bitcoin for itself. Any Bitcoin Gold will remain securely stored on Coinbase. If Coinbase were to enable support for Bitcoin Gold at a future date, customers would be able to withdraw Bitcoin Gold associated with their Coinbase/GDAX Bitcoin balances at the time of the Bitcoin Gold fork.
Link Here ===> https://support.coinbase.com/customer/portal/articles/2892196-bitcoin-gold-faq
More about the exchanges and BTC-GOLD