ApplicationGateway vs DefaultEventGateway

Hi all,
  I'm in the phase of (deep) studying the Barracuda framework to learn a
lot (how much i can) about it to use it to its best.
As i've said i'm moving to it from Enhydra SuperServlet arch, and i can
say i definitely appreciate it more (maybe just personal taste)
Anyway i would like to ask which are the main differences and purposes
of ApplicationGateway and DefaultEventGateway since from what i can
read/understand from javadocs i can say ApplicationGateway could be used
to 'do the event trick' on dispatching event from upon HttpRequest.
And since they both implements EventGateway interface.

For instance why should contrib.dbroggisch use them both one in the
web.xml deployment descriptor and the other in the event-gateway
ApplicationAssembler xml file !?

Thanks for any hint

--
Thelmo