I understand how you feel, but I personally feel that gossiping about them back or doing anything mean to them is just stooping down to their level.
My suggestions are:
-Don't let it bother you. You can go home and cry about it later, but while they're around, pretend like it just rolls off their back. They gossip because they think they'll get a reaction from you. Don't give them one.
-Talk to them about it. Find them at lunch and tell them that you know what they did. Ask them why they did it, and why anyone would want to gossip about anyone. This takes a lot of guts, but if you really want to do something about it, the best thing to do is bite the bullet and confront them.
-Wait and see what happens. Maybe it was just a one-time thing. But if this escalates, or if they had done something like this to others, tell the Principal or your parents.
I hope I have been able to help... don't try to do anything mean is my bet because it's just as bad as what they're doing.
Hope I helped!