Microsoft offers a product that does something very similar to this. Their Desktop Optimization Pack has a component, Advanced Group Policy Management, which includes a full revision control process for managing changes to GPO. If even includes the ability to roll changes back to previous versions.
I bring this up because change-tracking a GPO is a hard problem. Values can change in the XML, whole files can change without reflection in the XML, and other hard-to-track problems. Microsoft solved this by forcing all GPO changes to be submitted to a central service, which they did by extending the GPO management tools.