Six years is a long wait for a sequel, but it was well worth it. In that time, Valve created their own game engine, known as Source, and with the Gravity Gun quickly became THE game to copy. Like ClassicDOOM before it, most FPS games that came after Half-Life 2 valiantly attempted to copy the in-game physics engine that allows to to catch and repel in-game props. Many of them succeeded, but Half-Life 2 was the first to do it, and gamers know where to give credit where credit is due.
Once again, you play as Gordon Freeman, mute savior of humankind. Reawakened by the G-Man from the previous game "some time later," you find that the world as you know it is no longer. The Combine have successfully invaded and subjuated humanity, and it's only a matter of time before Man becomes just another X-stamp on the Combine's evolutionary whiteboard. Unless you can rise up to the challenge of liberating your very species, that time will approach far quicker than you'd like.
One of the first FPS games to introduce drivable vehicles, adding the Gravity Gun is just icing on this cake (that's a metaphor, not a lie). Chock full of references and easter-eggs, Valve should be proud that their title has continued to successfully carry the FPS banner.