Why Vala? GTK2 will be deprecated soon, and wxGTK will need some time to be updated to GTK3. Editor is not mature, only the UI is made.
Vala is a language similar to C#. It’s compiled into C, so it’s damn fast. It uses Glib (GObject) to do this magic. And, of course, it’s portable.
Is fast to develop, and you can debug it with GDB debugger because it generates C code, GDB can work with Vala breakpoints without problems.
Vala kickstart example with GTK+:
people.gnome.org/~aruiz/videos/g … t-vala.ogg
Parasite, “firebug for GTK+”:
chipx86.github.com/gtkparasite/v … -intro.avi
And of course, you can use Glade .ui files with Vala.