Apple's General Midi quality vs. M$'s GM

General Midi comes in different flavours (1 & 2) as well as manufacturers' own enhanced versions that aren't fully compatible with those of other builders of mid sounds. Midi is simply control numbers coming at an extremely fast rate to generate a digital sound. You can buy digital sounds and install them in your computer. Windows sounds are a basic set (GM 2) that are pretty good if you don't listen critically to midi. Apple's are probably newer and may be more expensive to license. Roland makes an extremely good set of sounds and enhances them with their own versions as well.
As an example... a basic nylon string guitar sound may not be "velocity sensitive", therefore it will always sound at the same volume whether you hit your keyboard hard or softly. A better sound will respond to the power of your keyboard tap, but Roland has a nylon guitar sound that also pulls the string at first (if you hit it hard) and follows with a small vibrato after a few moments. You get a much more realistic sound.