Are you sure it's just not your perspective? Try tilting the screen forward and back slowly, and watch if the gradient changes position. If it does, it's probably just the way you're looking at the screen. Imagine if your LCD display was made out of millions of tiny, tiny little Hollywood Squares that change the color of the light source behind them. If you look at the Squares from an angle, you'll see more light through some Squares and less through others simply because of your perspective. Looking at them head on is best, but since the Squares above or below you are at more of an angle than the ones straight ahead of you, you'll end up seeing 'brighter' and 'dimmer' squares no matter what.
All LCD displays have this side-effect, since the light getting to you is obscured in places (as opposed to a CRT, where it's all projected on more-or-less clear glass), and I think the glossy screen just makes it more noticeable than on standard matte screens.
