What questions are prospective employers allowed to ask past employers?

I worked at a small company that calls themselves a dsyfunctional family. Gossip and troublemaking is widespread at that place of business. Alot of the people causing the trouble was some of the top people. I complained a few times and found it made it worse on me. I stopped complaining and after a few months I just quit. I worked there for two years and if I don't include them on my resume then I have a two year empty space on my resume. Any information will be appreciated.