Emmanuele, I mentioned update-alternatives for the gedit binary name. For the GtkSourceView library, there is of course the parallel-installability mechanism which permits to the different major/API versions to co-exist on the same prefix (the same as used for GTK 1, 2, 3 and 4).
You seem to confuse different things. First, I published my branches and git repos on GitHub, but the license doesn't force me to, if it's for my own use and the software doesn't get distributed outside an organization. Everybody, every organization is able to modify free software for their own uses. It's just that with a copyleft license, IF the modified piece of software is distributed outside the organization, then the source code needs to be provided under the same terms (to simplify).
In short, that's the difference between proprietary software and "private" software (for our own uses / modifications that don't go outside the organization).
And, legally, I'm in my right to re-use (currently and AFAIK), the gedit and GtkSourceView names! There is another, different, legal mechanism for protecting that.
So in short, I'm in my rights.
To make several versions of gedit parallel-installable, it's just a matter of renaming a few things that get installed, including the binary name: it could be called gedit3 or something, and via update-alternatives or a simple bash alias, a user can "bind" gedit to gedit3. (update-alternatives is also used by SUSE, BTW).
Michael, what about Enchant, it is also developed on GitHub, outside the gnome.org infrastructure, or the GNU libc, or the Linux kernel? Which GNOME all depends on.