From GDAX website:

"All GDAX users with a BTC balance at the time of the snapshot will be credited an equal amount of B2X. Users do not need to take any action and will automatically be credited with B2X into a new B2X balance in GDAX at the time of the fork."

I assume it's the same for Coinbase. The fork supposedly happened today at block 501451.

https://support.gdax.com/customer/portal/articles/2893011-bitcoin-segwit2x-fork-faq