The game was first released in May and got tons of great reviews for its impressive single-player campaign, along with its easy-to-use SnapMap level creator. Since then, id has updated DOOM, adding in some new multiplayer modes for free along with some SnapMap improvements and additions. It recently released its first of three multiplayer DLC packs, which adds three new maps, a new playable demon, a new weapon and more. Id has hinted that more single player content could be coming in the future.
By the way, GameStop's discount is for the disc version of DOOM, in case you want to go old-school with your game purchases.