Documentation on the GAUGE API can be found here:
You can also download the complete documentation in the following formats:
Note that this documentation is not complete. However, many of the classe which are not documented are classes which are going to be removed and/or redone soon anyway.
Also note that GAUGE is far from done. There is still a lot of functionality that GAUGE doesn't have. These documents are here to give you a rough idea of how GAUGE works and some of the stuff that it can do.
If you have any questions, feel free to e-mail me.