GTK is the toolkit used to build applications, the gnome desktop environment uses GTK. So a GTK application is simply an application that uses GTK, all of GNOME is a GTK application, the system-config tools are GTK applications as well.
A GNOME application on the other hand, is an application that uses GTK but in addition it is made as a part of, or especially for use in the GNOME desktop environment.
GTK is the toolkit used to build applications, the gnome desktop environment uses GTK. So a GTK application is simply an application that uses GTK, all of GNOME is a GTK application, the system-config tools are GTK applications as well.
A GNOME application on the other hand, is an application that uses GTK but in addition it is made as a part of, or especially for use in the GNOME desktop environment.