GTK+ is the primary library used to construct user interfaces in GNOME. Its name stands for "GIMP Tool Kit", as originally it was explicitly written for that image manipulation program, and later extracted from it as a stand-alone library. It provides all the user interface controls, or widgets, used in a common graphical application. Its object-oriented API allows you to construct user interfaces without dealing with the low-level details of drawing and device interaction.

In addition to basic widgets, such as buttons, check boxes, and text entries, GTK+ also provides Model-View-Controller (MVC) APIs for tree views, multi-line text fields, and menu and toolbar actions.

Widgets in GTK+ are placed on windows using a box-packing model. Programmers specify only how to pack widgets together in container boxes, instead of positioning them directly with absolute coordinates. Thus, GTK+ ensures that windows are sized correctly to fit their contents, and it automatically handles window resizing. For right-to-left languages like Arabic and Hebrew, GTK+ automatically reverses the user interface from left to right so that controls have the expected visual order.

GTK+ allows you to develop custom widgets for use in applications. Similar to native or stock widgets, these custom widgets can support all the features that GTK+ has: right-to-left language support, accessibility interfaces, keyboard navigation, and automatic sizing.