What is a gem promo?
They have a Trine promo that is pretty neat. They have updated Trine 1. If you already have it, you get the updated version free.
A gem promo is just what they call a sale on a single game. It used to be called the "hidden gem" promo, supposedly to highlight a good but lesser known game by providing a discount. They then started calling the promo "indie gem" or "classic gem" based on the type of game it was, but now it's just called the gem promo.
The Trine promo isn't exclusive to GOG, but is also available on Steam and Humble. Basically, if you bought the PC version of Trine somewhere, you get the upgrade to the Enchanted Edition for free.