So i just bought the season pass for the Witcher 3 from G2A during the Black friday special. The problem i have encoutered is when i try to redeem the code an error comes up that says "some of the content requires the base game" which i already have. i bought the boxed game of the year edition of the game and activated it on GOG without a problem and ive played through the base game twice. so i do not understand the problem here. please help.

Contact G2A. The code may have already been used, or it may have been for another platform (Steam?)

If you bought the Game of the year edition (as you say), you already have the expansions. No new code needed.

If you do not have the game of the year edition:
"The boxed PC version of the game is compatible only with the Expansion Pass sold on GOG.com"

So, as previously stated, your key is not from GOG or is already taken.