The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
GUIKit Example: SimpleDialogApp
This example illustrates how to use modal dialogs initiated from another window, optionally setting up property values on widgets in the modal dialog and, when the modal dialog is closed, retrieving values from the widgets in the modal dialog.
Note: This modal dialog will be modal to other user interface definitions, but not modal to interactions with the Wolfram Language or the kernel. Modal interactions with the kernel are determined by using GUIRun or GUIRunModal.