Typically a gallery gets 40%. I have heard of some artists getting commissions through galleries, but not many. It seems that the galleries don't want to lose their control of the customer contact information and feel that the artist is likely to cut them out of their percentage.
As far as what you'd want to know about any gallery, I'd recommend reading Calvin Goodman's excellent art marketing book. I'd link to it on Amazon but they ask for one to two months to order it. You can get it through the author via this link:
http://www.portraitartist.com/resources/consultants.htm
It costs $65 but is a very fat book and is worth its weight in gold for art marketing and business advice of all kinds.