do i have to buy Darksiders Warmastered Edition again to have it to download?

what do you mean again? Do you own it here?

If you own it here, on GOG, you just go to your account and click on the game's card. There you have two options:

1. use the GOG Galaxy to install the game, or
2. download the Offline backup game installers and install the game manually after you download all 7 parts.

See the attached screen-shot.

If you bought it on Steam or a code for Steam you have to download it there.
If you bought it here, just follow the the instructions above.
If you got a code for GOG.com, redeem it here: https://www.gog.com/redeem And then follow the instructions above.
If you bought a code for here, but you bought it somewhere else... it might be that you've been cheated. GOG codes aren't usually sold outside this very store. Many GOG keys that show up on 3rd-party sites were purchased with stolen CC info and probably revoked.

It never hurts to try, at least!