From their website: (Pls check their website {status.gdax.com/} yourselves.)
We (GDAX) will be performing scheduled maintenance on Wednesday, January 31 from 9:00 PM PST to 11:00 PM PST. All resting orders will remain in force. Customers will be able to cancel resting orders prior to live trading resuming. Full Schedule: 9:00 PM PST: GDAX API and website will be offline. 11:00 PM PST: All markets will enter post-only mode for a minimum of 10 minutes. Post-only limit orders will be accepted, but no matches will occur. You may also cancel any resting order during this period. 11:10 PM PST: All markets will enter limit-only mode for a minimum of 10 minutes. Limit orders will be accepted and may be matched. Market orders will not be accepted. 11:20 PM PST: All markets will enter full-trading mode