I have some good news, and I have some REALLY good news.
@Asuka_Kurashina Gmod compatibility has been fixed
Black notes are finally working! Yes, you read that right, I finally managed to find a solution that allows us to play black keys!
Do note that this is an experimental release and it might have some bugs. It should be stable, but some functionality is lacking. At this time, you can't play black keys that are symbols, such as '!"#' etc. But that will be fixed in a later update. Any other black key should work.
Please let me know if you find some functionality and/or black keys that are not working. This is a big breakthrough for instrument autoplayers.
Without further ado, the download link
In case you are looking for the stable release the link for that is here
Do note that this version does not have the ability to play black keys and does (apparently) not support gmod piano.
And if anyone is interested in the "behind the scenes". The source code for the program is available on github here. Master branch contains the stable version and Experimental branch contains the latest experimental version.
EDIT: It seems I've managed to break save and load. Woops.
EDIT2: Version 2.2a is uploaded, fixing the save and load issues. Same link