The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
These tutorials help you to start to use GUIKit to develop your own user interfaces. It shows how to launch and run GUIs, how to inspect them as they are running, and how to control their behavior as they run. It ends with a description of the basic components of a GUIKit definition.
Other tutorials help you with more advanced topics such as building AddOns with GUIs and building libraries of common GUI components, as well as more detailed information about scoping for scripts and names.