The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
Code
Wolfram Language code: Needs["GUIKit`"]Wolfram Language code: TextEditor[] := GUIRun["Wolfram/Example/EditorApp/Editor"]
Example
This example shows how to design a user interface from multiple definition files by loading each one with relative path references. See the code definition file (Wolfram/Example/EditorApp/Editor) for details on the implementation.