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.
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.