The functionality provided by GUIKit has been superseded by the interface construction
functions native to the built-in Wolfram Language.
GUIKit Example: Hello World
This example displays the classic simple "Hello World" application using the GUIKit framework.
Optional Additional Forms Achieving the Same Result
The following are other variants on the basic theme of specifying the user interface definition.
Here you choose to pass arguments to the constructor of the label widget instead of setting the "text" property.
Technically, even this version will work since frame and content panes will be automatically wrapped if not included.
You could add extra layout options to center the text.
The "Panel" and "Label" widget controls simply wrap the Java Swing controls, which you can also call directly through either the Swing or AWT versions.
Here we also try this using an XML definition equivalent.