The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
GUIKit Example: PrimeFinder
This example displays a dialog for finding prime numbers and alerting the user when any entered expression evaluates to a prime number. The modal version also shows how results can be returned to the Wolfram Language when the dialog is finished. In this example, a sorted list of all prime numbers visited during the session is returned.
This dialog can also be run in a modeless session as well, with multiple copies running independently.