I see this user has not logged in since 26 may be hopeless but I see a glimmer of light here.
GDAX is a famous platform, so their reputation will be important for them.
It's a huge amount worth fighting for, raise it up on Reddit and if you're lucky it will be resolved quickly.
Here are these quotations of a similar problem.
"Unfortunately we are unable to recover coins that have been sent to an incorrect address. The support overhead created by doing so is currently unmanageable and this is not something we are able to do. Please be careful in future when depositing coins to ensure they are being sent to the correct coin address. Any questions please get in touch."
Hi depboy, Yours and other tickets have prompted a discussion internally shortly after I answered your first ticket. I did feel uncomfortable, given the amount, telling you it was unrecoverable but this was our policy at the time. I have raised the idea of doing recoveries over a certain value for a fee. Or charging a flat fee for any recovery. Our support lead is away today however they and our business manager have plans to discuss this tomorrow, being August 9th in NZ. I will make sure they are aware of your case and we will pass on any updates as soon as they become available. In the meantime if you have any questions or concerns please let me know. Thanks, Jerry Cryptopia Support
Hi depboy, First up, let me apologize and explain what happened in this case. The secondary wallets we use to recover coins with were re-purposed for the sake of splitting our BTC to BCH and weren't in a state to do recoveries. When we were queried if we could do a coin recovery for BCH, one of our admins said "no, we can't" because our systems to do it were and still are a bit of a train wreck; Jerry, god bless his soul it wasn't his fault, took this as a hard no, rather than a "come back in a while" and responded to you in kind. The template you received was one we send to people that ask us to recover coins where there is absolutely no economic sense to do so (aka spending 15 mins to recover coins worth 10 cents). Secondly, the rock and the hard place being that it's going to be a few days to recover your coins, and we're not gonna start until next week (yay growth?) because we're busy, but I do accept us as a company are responsible for our internal miscommunication that caused this drama. I don't want to put your through the drama of updating a support ticket every day to see if today's the day we found time to sort this out for you. As we know the coins are completely recoverable, and I really don't want to set a precedent for this, but I'll make an exception - I've used our user transfer system to send you 25.57385629 BCH, and changed your recovery request to credit them to me instead. This means that you won't be out of pocket until we can be bothered recovering them, and if we take a month to recover them that becomes a me problem instead of a you problem. Sorry again for the dramas, I hope you enjoy your time on Cryptopia. If you have any questions, queries or feedback feel free to get in touch with us. Cheers, SeriousFace The Cryptopia Team
Source:
https://www.reddit.com/r/btc/comments/6s49ql/psa_careful_when_sending_bccbch_to_an_exchange/