When I use to be a LO for a local credit union. The max the dealerships could charge for gap products was $700, if you have a local instution that you aer financing through ask them first -- the CU's I worked for were ALWAYS less expensive than the dealership. Also, look into see what your auto insurance offers as well 
We were able to add GAP insurance to my husband's vehicle because I know we are upside down and I want to be protected rather than in trouble if something happened!