This is bizarre - the exact same thing happened to me, and the email contained that same address. (That's how I found your post; pasted from the email that Coinbase sent me: 5a31b72f0bc41d0221e844ad. That's at least three people who got sent that same address.) I sent it from an external wallet to my GDAX wallet, both proper BCH addresses, and the email from Coinbase has this same unknown 24 character address. It's also been 4 days for me now, it hasn't arrived in my GDAX wallet. I'm trying to get help from Coinbase but they don't seem to understand the problem (said something generic about blockchain transactions being irreversible).
I wonder, was this an internal Coinbase glitch, perhaps introduced during the BCH craziness on the 19th? Or was Coinbase compromised so that multiple BCH transfers ended up in that same address? I would both like to know the answer, since it's an indication of how safe it is to use Coinbase; and I'd like for all of us to get our money back. Coinbase support hasn't been helpful so far.
If anything happens I'll post an update. If you could let me know about your progress I'd appreciate it!
RE: Strange Issue when Attempting BCH Withdrawal from GDAX