Hi, does anyone know how to solve that problem. I have a gift code I want to redeem for a grim dawn expansion ( Ashes of Malmouth) that is redeemable on GoG, but the original game is installed on steam. I connected my steam account with a GoG account and the game was recognized. Whenever I enter the gift code it says however, that a base game is required.

Thanks for your help

You can't mix and match games and DLCs from different stores, that's how it goes.
You couldn't buy DLC from Steam if you have the base game on GOG either.

So the only solution is to choose one store, and then buy everything from there.

What PixelBoy said is true: You have the base game on Steam, so you need to buy the DLCs for your Steam installation.
In your case, you needed an Ashes of Malmouth DLC STEAM code.

I suggest you do one of the following:

1. Try to get a refund from the store you bought it from, or
2. Buy the base game on GOG, or
3. Find someone willing to trade your DLC code for something else you want

Ok, thank you you two for replying. That is annoying but my own negligence. I will try to exchange/refund the code with one from steam if the seller agrees.