This is not a Steam key, please do not activate it in Steam.
Once you receive your key, you can activate it in https://socialclub.rockstargames.com/activate. In order to do that, you must log in to an existing Rockstar Social Club account or create a new one.
After activating, you can download the game via the setup tool provided by Rockstar.
Grand Theft Auto V, commonly shortened to GTA V, is an upcoming open world action-adventure video game being developed by Rockstar North in the United Kingdom and published by Rockstar Games.The game will be the first major title in the Grand Theft Auto series since Grand Theft Auto IV