Gwen is Francine's adoptive sister. Stan finds her very attractive. Her parents don't believe that she can be successful on her own, so they leave her all of their money in their will. They say it's because Gwen is not intelligent, due to failing math. However, it was revealed in "Now and Gwen" that Gwen was able to cleverly sell counterfeit cell phones for years without her parole, or anyone else knowing. After truly seeing how smart she is, many fans speculate that Gwen either failed math on purpose or just didn't bother to succeed in it.
Besides Gwen being shown to be slightly more self centered and greedy than Francine, she does seem to be better than her at everything. Gwen is far more intelligent and popular than Francine.
She made her first appearance in the 11th season where she stayed with Francine to do her criminal business and avoid going to prison for avoiding her parole by leaving the state.