I think it's best to start by doing some homework. Find out (as you may
have already) whether or not the gallery generally shows both photography as
well as the kind of work that you do. If they do, call or write to them to
ask if you could make an appointment to show them your work.
If they agree, I don't think it's necessary to have the prints matted or
framed, but by all means they should look as good as they possibly can, and
they should be at the size you plan to exhibit. If the edges/borders of the
prints are dirty or uneven, then you might well mat them.
Don't make it hard for them to see the work by putting in too many sheets of
interleaving tissue, etc.; you want to be able to put the work in front of
them and allow them to see if it's something in which they have any
interest. I would also suggest limiting the images that you show them to
one particular body of work, and not to have more than about 30 pieces.
I think it's best to answer any questions they may have, but not to talk
too much while they are looking at the work. It only distracts them from
the images.
Most people who look at art for a living will know pretty quickly if they
are seeing something they like. Don't be put off by what might seem like a
fast-paced review, and don't necessarily expect a critique of the work.
Good luck!