This is an interesting question.
I have tried to browse through the https://www.midi.org/ (Trademark policy, IP policy and ToS) and I formed my opinion based on those.
There may be two problems for you.
Trademark. Since "MIDI" is trademarked, you can only use it according to its usage policy. While it is allowed to use even commercially (the website is rather liberal about its use), it is only valid for products working completely according to the official specifications. If you strictly follow the specification then the usage is valid; if you deviate from the official spec it could be make valid by specifically saying that your work is based on the official, but since it deviates it is not "MIDI compatible" and cannot be called "MIDI", but it does not prevent you from mentioning that.
(It may be tricky to name your repo, to avoid confusion between "midi" and "midi-like".)
Copyright. While databases may be copyrightable as a whole this whole spec is a creative work, so it'd be copyrighted by itself (you canot come up with the same numbers by yourself). However all the website suggests that these are specifications for the general public (they are even referenced or partially contained by other, open and public standards) and you are allowed to use it provided it's aligned with the Midi Association's plans and purposes (which seems to be to distribute the real and reliable standards).
So, I would say that if you strictly follow the GM spec then it is valid for both the ™ and the ©, and if you deviate then you shall not call it "MIDI" but you can mention that it's based on that.
It is probably useful to repeat these sentiments in your shared repo as well if you apply them.
(If you're really worried you could ask them directly, I would if my project required to make it sure.)