It's a great idea as it tempts me to support GOG and Developers by purchasing game types I dislike (e.g. rogue) or others I'd never play (e.g. cause they're boring).

Just one small thing though ...... May be worth adding some way to upgrade the version offered.

As a totally random example lets say I receive a 15% discount offer on "Pathfinder: Kingmaker - Explorer Edition". This is the type of game I once loved but now, thanks to job & family, cannot ever hope to play past the first stages - hence it's a waste to purchase. That said there's normally a variable financial discount marker I set on all games (except those I consider unethical) which motivates me to make a purchase to show my support.
The problem is that "Pathfinder: Kingmaker - Explorer Edition" is the base edition and there's no way to apply the discount to "Pathfinder: Kingmaker - Imperial Edition", nor is there an upgrade option available outside the discounted purchase in the general store.
I get more emails in a day than I have time to read so most are binned, this often includes GOG sales since I'm always receiving false listings and incorrect information. These "Hidden discounts" will also end up in the spam folder if we can't find a way to stop me receiving "deals" on a base game with no upgrade options or a base game where I already own the upgraded edition.

Just food for thought.