There is no other documentation than the one I pointed you at. The best documentation is the code itself.
At a high level:
* the agent is written mostly in groovy and uses restlet to expose its (rest) api (which under the cover uses jetty)
* the console is written mostly in groovy and is a grails application (both web and rest api are exposed via the same grails app)
The documentation I pointed you at describe briefly what each directory contains.
Before you dive into programming glu you should make sure that you are very familiar with glu and all its concepts.
You should also ask yourself what you are trying to achieve before doing it. There are other threads on this forum asking similar questions. glu is pretty flexible and extensible and before jumping into changing it, you need to know deeply how glu work as a user first.