In order to redeem your key follow the instructions below:

Go here and log into your Rockstar Games Social Club account. If you don't have a valid account yet, join the Club!
Enter your code on the Social Club Activation page and download the GTA V PC Setup Tool. 
Download your game and enjoy!

In case of any issue with game download please follow the instruction below:
Once connected, you should click on the cog at the top right corner of the screen, and on "download games" in the menu. Then You will be able to click on "Download for Windows".