The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
Interface definitions can be defined using either a Wolfram Language expression or an XML definition. For many usages, the Wolfram Language syntax is preferred. However, in cases where you want to launch GUIs before the Wolfram Language has been launched, the XML definition is useful.
Import and Export
You can work with the XML definition for GUIKit interfaces from within the Wolfram Language using the Import/Export format type "GUIKitXML". The GUIKit` package adds support for this format type so that you can convert between the Wolfram Language and the XML formats.
Knowing that $GUIPath provides access to all your interfaces, you can use this to easily import XML definitions into the Wolfram Language as the equivalent expression format.
Here is what the contents of the XML file looks like for your base frame definition.
Here is that same definition imported as the Wolfram Language expression.
You can easily switch between the XML or expression format and can even use the SymbolicXML expression in your calls to GUIRun or GUIRunModal.
Here is a screen shot of what the previous definition renders.
The GUIKit deployment functions can also take the SymbolicXML definition forms as direct input.