Market numbers do not validate or invalidate an opinion. If you don't like gears, nobody can argue that, because it's true.
Market share, particularly in the video game sector, does not dictate which product is superior. It dictates which one appeals to a larger base. Halo is nowhere near the quality of UT, not even close. But it was released on a console that was very aggressively marketed to a very large audience that would not play PC games but would play console games (college frats, mostly). The XBox was more convenient for more people than PC was, and less expensive most of the time, so it reached a larger base. And console gamers who had never seen a good shooter before thought Halo was the best thing ever, and so it sold more than UT which existed in a considerably more niche market. Consider, for example, that at one time the primary market for gameboys was middle-aged women. Consider the size of that market, and then consider the amount of market share PCs have in that particular market. If a game is available to more people, of course it will sell more, and not because of it being a superior game.
Gears ran fine on my system, and it's not exactly top of the line. Yeah, there are issues, but that's the case with every PC game ever released -- remember the problems Steam had with HL2's launch? The audio stuttering issue and things like that? And yet they're still wildly successful.
Gears just doesn't hold up to most PC games that it directly competes with. It's not a bad game by any means, but it's fairly monotonous when you get right down to it, and the boss fights are really the only things that stood out about it. The standard level play, which is 90% of the game, was boring after the first couple of scenes. PC gamers expect more than that, and so it received a lower score for the PC. But this is nothing new; console games generally tend to be a lot less complex and engaging than PC games simply because of the limits of console platforms. You would never find the original Mechwarrior games on a console, for example (Mechassault was a very pared-down version of Mech4), or any of the great space sims of their day. It's just not what consoles do best.