Biggest in the sense of sales, size of the game or development budget? GOG don't really publish their sales numbers, and budgets are often relatively private too, unless it's a crowdfunded project. But I'd guess it's probably The Witcher 3 which is quite a big game in many senses.

From the recently released, Shadow Warrior 2 is also pretty big. I wouldn't call the above mentioned TToN a small game either.

Witcher 3, big AAA title about the adventures of a magical swordfighter who also gets laid. The twist is, the developer also happens to own GoG.