I too have the exact same issue with the exact same game. I had 8 different other games 'connect', but ZNT just won't... :/
..Found this at the bottom of a support page: (support.gog.com/hc/en-us/categories/201552985)
"Some users have reported that launching the game via Steam helps if it is not listed as "available" in GOG Connect.
If the above doesn't work, even after a few days, please contact us, by providing a direct link to your Steam profile, and a screenshot of the gog.com/connect page."
...Hope that helps
**Afternoon Update**
I contacted support about the issue and once they had re-synchronized my accounts at their end (which needs 2 hours before trying connect again), it worked. Happy Gaming, people!
Post edited December 13, 2017 by CarbonDevice