It's incredibly difficult to write a review of a game where any serious discussion of the plot or mechanics ruins the experience. Gravity Bone is a graphically simple 1st Person perspective game that is shockingly good. The Minecraftey NPCs, the lack of setup, and the reveal all serve to play with your expectations of what the game is, or is supposed to be.
Simply put, Gravity Bone is best when you know nothing about it and are simply taken along for the ride. It's a spy game, a platformer, an adventure game of using and combining items and yet even then it is ultimately about none of those things at all. It's free and runs well even on old hardware so there's no excuse not to experience it yourself. Just be prepared to be obtuse when someone asks you what it was about. Do yourself a favor and read nothing about it before playing it, letting the in-game cues inform your expectations. Then, when you finish it, come back and thank me.