There are fewer pure C GUI libraries than C++ ones, but I would not bother to learn C++
just to do GUI's.* I've done tons of stuff in C with gtk, including stuff now distributed by debian. WinAPI is vanilla C too, I believe. Most of the linux desktop is C/gtk -- it ain't C++, this is a lot of whining. WRT to gtk, the results are the same, just the C++ libs (gtkmm) use 200-300% more memory (but your codebase will be smaller).
Glade is like a scripting tool for use with Gtk.
Skip it. You might as well just focus your energy on Gtk itself:
GTK+ 2.0 Tutorial -- what I started from
GTK+ Programming -- very helpful forum, mostly C but C++ welcome too
GTK+ Reference Manual -- the API reference proper. Very tough at first!
Which implies something doesn't it? If you bite the bullet and learn the API, you won't have to depend on glade, and in the end you'll have a better understanding of what you're doing and how you can exploit it.
* if you were going to learn a language just to do GUI's, I'd skip straight to python or perl, etc.