As my review of Runner suggested, perhaps the effective way to create emotional meaning in games is through metaphor. Gray is all metaphor, and is interesting precisely because it is. But it's also, well, very tedious to play. Which raises the question of whether art is effective through a metaphorical level even if it is, in some sense, bad craft.
In Gray, you are a small black-tinted person in a field where dozens of white people are running left-to-right past you. Some of the white people are black-outlined; you can maneuver yourself to them, and press "space bar" to enter a "dialog" with them. The "dialog" is represented by a wave that propagates left-to-right from the other person's side of a speech baloon; your objective is to hit space-bar at the right moment so that your speech wave meets theirs "in the middle" (metaphor alert). If you succeed, the dialog goes away, the little white person clutches his head as if in thought, then turns black and turns around to run the opposite way.
Eventually, everybody is black. At this point, you clutch your head as if having an idea, and turn white. Now, you have to -- yes, you get the idea. This time it's harder, however; before the first mass-conversion, you need to hit space-bar only once, or sometimes twice, and time it correctly to convert. The second time around, your opponents typically send three or four "waves" at you, and your "responses" must all be timed correctly. So the difficulty increases.
There is an ending, which also fits into the metaphor, and I'll discuss that after the break, in case you hate spoilers. But I can certainly discuss the game's essential problem before then. It is, as I've alluded, that the game is tedious. Gameplay is maneuvering yourself to intercept someone you can talk to, and then pressing space-bar at the right moment in "conversation." And it's the same basic gameplay over and over, with minor variations for conversation timing. Dull.
Notably, the second time you need to engage in fewer conversations to pull off mass conversion -- perhaps an indication that the developers recognized that their game was dull, and decided to short-circuit the second and subsequent levels in the hope that you'd persevere to get the "ending" despite your boredom.
In short, Gray is making an effective point about the nature of political debate and what seems to be an uncomfortably widening and irreconcilable split in our society between left and right. It is somewhat effective at getting across this point, and the black-and-white nature of the graphics is arty and effective as well. Thus, it's worth playing as an interesting example of metaphorical meaning in gameplay -- but it's still, when you come down to it, pretty tedious.
After several flips, you become gray. And can no longer persuade anyone. I guess you've achieved satori-like enlightenment, but unlike a true Bodhisattva, you cannot radiate your enlightenment through the game-space.
Which brings up the question of the value of Gray's metaphor. It is not, in fact, true that truth lies "in between." Truth does not lie "in between" those who believe that the Earth is flat and those who do not. Truth is "justified belief." Some things are true, and some things are not. Some things are moral, and some things are immoral; no "new idea" will ever justify murder.
The ending itself, in other words, implies that moral relativism is "truth" -- which is untrue. In my opinion :).