I don't understand what this program is good for. If it just divides the existing pixels up into more pixels without really changing the resolution of the game, what's the sense?
Well, GSM just makes a simple upscaling. It doesn't making interpolation (i.e. it doesn't add extra pixels / lines). So it doesn't increase the internal(=original=source) resolution, only the output (=forced=target) one.
So, there is no miracle here... The greater the quality of source (original) resolution of the game, the better will be the results that will be displayed on target (forced) resolutions - specially on the higher ones, where the images naturally tend to be pixelized. BTW, see "List of games using your PS2 to the maximum" topic below.