Yeah plenty of Launchers that do that, and each has their own download system… At least Modrinth is allowing GitHub Releases to be downloaded, so your Mod is fine already. 
I already posted all source code that i did have from those older versions on my site, so if it isn’t there, it simply does not exist anymore and is lost forever. The reason it was lost is that until mid-GT6 I did NOT use git at all, and i always overwrote my old code with the new code for the next minecraft version.
The first Onebox has the “GitHub Issue” Icon, since it is posted as an “Issue”, the second is the one of the actual project.
Edit: Anyways, time to get the other 5 GT versions up on GitHub.
Edit: Still need to do Releases there but at least most of the repetitive junk has been done.