Hi , I recently bought a game Witcher 3 : Wild Hunt . I can not install because my pc over tighten it . When i got a new graphics card i install witcher . But after writing Gaming code gog.com me writes : Code already used. Please help... Thanks

you must login on account on which you have activate witcher 3 one code cant be activate on two account

I have only this acc.

if you have only this acc then you dont need activate witcher 3 again you should have witcher 3 in you libray already

If you need help downloading your games read this FAQ:
http://www.gog.com/support/website_help/downloads_and_games

I have witcher installed but i mean active him, but when i write code, i get answer: Code already used.
If this help: I bought game in supermarket, no in gog shop/steam.

Then you should ask in the supermarket for a new game because your code is invalid. Or you can write to GOG support and see if they can help you: http://www.gog.com/support/contact
Good luck!