GTK+ is written in C, although it is designed within an object-oriented paradigm (unlike C++, C does not support object-orientation natively). The toolkit offers bindings to almost all popular programming languages.
The look of the toolkit is largely configurable by the user, right down to offering a number of different display engines. There are engines emulating the look of other popular toolkits or platforms, like Windows 95, Motif, or NeXTStep.
The GNOME environment uses GTK+ as a base, which means that programs written for GNOME use GTK+ as their toolkit. GNOME applications are not the only programs using it, though, and any GTK+ program (or GNOME program, for that matter) can run on top of other desktop environments like KDE.