As far as I can tell, those projects I mentioned are all either maintained by GNOME developers under the GNOME umbrella, used in a large number of GTK apps, or they use glib/gobject and make extensive use of the same design patterns as GTK. It's confusing to me how you're trying to say they aren't designed to work together. The situation is the same as Qt, the only difference there is that it's centered around their C++ QObject design patterns.
If the argument is that the Qt libraries are all developed together by paid Qt Company developers then sure, that's true, the libraries I mentioned aren't like that where one company is working on them. But that's different than saying they're designed to work together.
If the argument is that the Qt libraries are all developed together by paid Qt Company developers then sure, that's true, the libraries I mentioned aren't like that where one company is working on them. But that's different than saying they're designed to work together.