Can't help with the SW aspect - but I too was a career switcher and it was a transformative experience.
My experience is that we specialize post masters - the grad school experience is just an intro to so many different ways of working and so many different populations.
I have a couple of friends with the BSW who work in "nursing" homes / assisted living homes and they ADORE their work and find it very satisfying.
As far as getting some current volunteer hours in - if your interest is geriatrics - it should be easy to get some volunteer time in at a senior center, meals on wheels, assisted living etc. Offer to lead a group, facilitate an activity, teach a skill, or just be a listener to someone who is alone.
With you interest in working with older gay folks - I would start out with making connections with your AIDS/HIV support groups/agencies and branch out from there.
There is plenty of room in the field of mental health for career switchers. Welcome.