https://www.gog.com/giveaway/claim
It will give a json response. Visiting a second time should return “Already claimed” if it worked.
This helped me to get around an issue I had with the link not loading on the main gog.com page when there was a free giveaway. (Credit)
(If you still have trouble, make sure you are logged in before clicking)
This is fantastic. I’ve never had a real problem claiming the giveaways before, but today I had the same issue you described, and I remembered this post. Thanks for sharing!