Ron is 100% right, Realtors are not allowed by law to designate an area as good or bad. A good agent, like Ron, will tell you that. A bad agent, like many ohers here in Trulia and many other sites, will try to steer you towards the area they prefer to work.
I am currently not an agent and can tell you that Gilbert in general is a very good area. Other very good areas are Chandler and Ahwatukee. Parts or Tempe, Mesa and other valley cities are also good. In generel you want to stay away from most of the west valley and most of south Phoenix.