GADX is their trading platform. They don't plan on supporting trading with BCC. That being said, no, you don't own a short position. I don't see why that would even be related to this.
Because shorting means you're borrowing BTC so you can sell it off. Since you borrowed BTC, you're the one who would have received the BCC for that BTC, so are you on the hook to give that back as well?
If I borrow your hen and it lays an egg and that hatches a chick, shouldn't I give you the chick back with the hen?
I just reread the thread and understand the discussion. I had misread the first post.
That being said, I think when margin trading on GADX no actual Bitcoin transactions are occurring. So, Coinbase is just doing accounting for the trades using internal assets. I think they could later on replay everything but treat all of the short positions as having exited directly at the split or something along those lines.
Here's Coinbase support posting a "how do I use GADX?" article: https://community.coinbase.com/t/what-is-gdax-how-do-i-use-i...