An attempt at a standard, which failed at some of its goals, mainly as an abstraction of (wavetable) synthesis.
You did not make music in GM for a certain device. If you did, you failed to understand what GM was all about.
Well no. GM was trying to be about something that just didn't work in practice.
You *always* make music for a certain device. That's the only way. You need *some* device to listen back to the composition you made, and adjust the volumes and sounds of the individual tracks to get the 'production' you are aiming for. In conventional recording, that is known as 'mastering'.
The problem here is that the mix you created on one GM device will not translate to other devices, because of subtle differences in both individual instruments, and also the mixing/compression algorithms used to mix multiple channels together to the final output.
In theory you could try your GM track on various devices, and try to find a lowest-common-denominator (I doubt that any composer actually went through that trouble). But obviously you can never tweak your track for devices that aren't even available yet. There's no way of knowing how it will sound on future devices.
It's a fact that many games sound much better with later devices, simply because they are better.
It's hit-and-miss. And it's also personal taste.
It's a fact that games will sound *different* with later devices. In some cases, some people may perceive that as 'better', in others, it may be perceived as 'worse'.
Insisting that the SC-55 was what the games were made for is like saying 3dfx games from 1997 must be played on a Voodoo1, not on a Voodoo2 or 3, which did not exist at that time - and ignoring the fact that they look better and run faster on a Voodoo 2/3.
That analogy is severely flawed.
Regardless of what video card you use, you will always use the same geometry, textures etc, because that is just part of the game content.
In the case of GM, the instruments are not part of the game content.
So using a different GM-device would basically translate to using different geometry and textures in your game.
MIDI is just a sheet of notes and the better the orchestra, the better the resulting music.
That is overly simplified.
Guess what? Real orchestras need to practice, and they need a conductor to create the proper 'production'. The conductor will 'translate' the 'sheet of notes' to his specific orchestra and venue (acoustics also play a role), also known as the 'arrangement'. In doing so, he will also add a bit of his own interpretation of the music.
So for example, perhaps he will use a different amount of violinists than another orchestra/venue for a certain part, to get it to sound 'just right' for his specific case. He may also instruct the players to play in a different way (eg focus more on attack, or play with more/less volume etc).
No such adjustments are done for MIDI. You are sending the exact same note info to completely different instruments, which will have different characteristics.