i'd recommend having a look through the GTK+ / Gnome Application Developement book online at
http://developer.gnome.org/doc/GGAD/ takes you through all the gtk+ basics and the more complex gtk based gnome specific widgets in gnomeui. I really like gtk+, works very logically and it's easy to use under c. i have a small number of reservations when it comes to using it in C++, but it's still pretty impressive, and gtk2 is just out now with an improved api and widget set. you might also want to look into glade for a really easy gui builder for gtk/gnome which can output code in c, c++, perl and a bunch of otehr langauges.