There is a help file for GAX/GAT which may be just not enough when you want to deep dive but it should be fine if you're just starting. Which reference to documentation have you seen that is not working?
Also, the GAX/GAT Hands On Lab (detailde word document + source code) is a good starting point of a simple guidance package. You can find an updated version bundled with the Clarius Software Factories Toolkit at http://softwarefactoriestoolkit.net.
Lastly, you have the software factories from p&p, looking at their source code is a good way of learning GAX, but I don't recommend this until you have read some other basic info before (as they're larga and quite complex in some cases).
HTH,
-Victor.