My prediction is that gtkmm will not be very relevant unless there’s a surge of renewed interest from the community. You could be a part of that. gtkmm and all of its underlying C++ bindings are almost entirely maintained by one person, and he’s wanted to hand off the project for a long time.
I’ve not seen many apps that use gtkmm. Nickvision apps like Parabolic are in C++ but don’t use gtkmm; they just use a custom C++ template that wraps the C API manually.
I’m pretty sure I heard someone else wanted to make a new set of C++ bindings but I don’t remember who or how that turned out.