If you're doing a weekend BMQ, yes you would still attend the weekly training nights. The extra 3 hours is nothing to worry about though, and would be considerably more relaxed than BMQ.
As for "what stuff gets done", that would depend on whether you are a new Private, or one of the trained Gunners. As a new recruit, you would participate in drill, lectures, etc as all of the soldiers would, but when the Gunners are working with the 105mm Howitzer, you may be watching and learning, or working on other jobs that need to get done.
That was my position from November '04 to the beginning of this summer, it's a good time.