No, sorry, that's not how library development works—and, in general, that's not how free software development works.
Linux distributions even have a system for the user to choose which alternative is best for his/her needs, with
update-alternatives.
update-alternatives is a Debian-only thing, and even there it's not even going to be used for every single package; "gedit" is not a generic command, like "cc". Plus, the alternatives system is not going to work for libraries. What happens when you add new API to your fork of gtksourceview? Or when you don't rebase your fork against the latest API from upstream?
But the development is done in the open.
I'd hope so, because you're constrained by the licensing terms.
In any case, GtkSourceView and gedit are GNOME projects; the former is still heavily developed, and the latter has a maintainer who took over after you abruptly left for the second time. Just because you maintained them both projects at some point doesn't mean you get absolute rights over them. If you want to develop your own version of gtksourceview and gedit, and you also want to distribute them, you get to fork and rename your fork.
While gspell, amtk, and tepl are projects that ostensibly you could call your own, seeing as you started them (though, without a CLA, you're also limited on your options on that front, given that other people contributed to them over the years), gedit and gtksourceview existed long before you started contributing to them, and your code contributions, while appreciated, are not enough to make them wholly your projects.