gtk+ is a GUI (graphical user interface) library, which some programs (gimp, gtkradiant, etc.) use for drawing GUI (they don't use that standard windows GUI as they weren't written only for windows, but for many platforms, and gtk+ is available for many platforms). So you need this library if you want to run such programs.
Don't know if the gtkradiant's windows installer provides also the gtk+ library or if the library has to be installed separately. Nevertheless, as NiTRoX said, you should check wich version of gtk+ is required and make sure that you have it installed.