I wanted to but the dlc for the withcer 3 and i was just wondering can it be activated thought the GOG?

I think you meant that you want to buy it. And with DLC you meant the addon (Hearts of Stone) and not the free DLCs.

If you buy the addon on GOG, humble store or Amazon you will get a GOG key. Don't buy it on key selling sites. GOG don't allow that and they will delet the game sooner or later from your account.
The only way to get a Steam key is to buy it on Steam or exchange the retail key on the Witcher 3 website. (Same for Origin.)

Im not gona buy the Heart of Stone from some web-site i would buy it from the shop i got witcher 3

The retail version contains a GOG key.

Thanks for info,i got dlc and it works fine the shop they sell at didn't bother to write it can be used on steam/origin and gog