The functionality provided by GUIKit has been superseded by the interface construction and controls functions native to the built-in Wolfram Language.
GUIKit Example: NIntegrate Explorer
The NIntegrate Explorer is a GUI that lets you do numerical quadrature using NIntegrate. It opens a tool that lets you enter an integrand, set a region, and then modify and change the various option settings for NIntegrate. The result of the computation is shown, along with a graphical display of the function evaluations and the input to NIntegrate that was used.