The functionality provided by GUIKit has been superseded by the interface construction
functions native to the built-in Wolfram Language.
GUIKit Example: Making Progress
This example shows how to generate a reusable progress bar panel and use it as a modeless dialog for displaying progress with your own Wolfram Language calculations.
There are three important objects in the dialog that have been registered with reference names for easy lookup. Here are the most common properties worth setting during the lifetime of the dialog.
Once the dialog is not needed, you can call ReleaseGUIObject to dispose of the instance.