Familiarize yourself with JMX. A good book to read would be Java Management Extensions by J. Steven Perry published by O'Reilly.
ugh. I realise I'm being contradictory again, but...
Honestly, I found the O'Reilly JMX book to be horrid. It was "MSQL/MySQL" all over again.
If I can throw in my 2 cents, I found Manning's "JMX In Action" [Sullins & Whipple] to be a much better book, with good hands on work and coverage through it.
It's probably a matter of opinion and learning style, but take a look at both of them [I actually bought both, Amazon had a 'buy both and save $x%" deal].
When I had to learn GBeans to convert the OpenEJB containers to the new GBean system earlier this year, I worked off the Jetty bindings that Jeremy Boynes had written. They were fairly simple and can probably give you a quick run down.
As always, many of the developers, esp. Jeremy Boynes, Dain Sundstrom and Alan Cabrera, can be good resources if you can manage to pin them in a corner for a bit to ask some questions.
I good discussion about the many source files that contain the name GBean would help many people understand the GBean.
As noted by Philip, reviewing existing code is good too.
-Brendan
