You may be the original owner, so you certainly should know your own property. Trulia, however, sources property information from the tax records of the construction provided by the original builder, McCar Homes, which all currently show that this property has three bedrooms, two and one-half baths. The one photo of this property that I can find online shows a property with a room over the garage with a standard window, a common floorplan in Gwinnett County. Are you capturing that space as an additional bedroom? If so,
1. Does it have a full bathroom or is it very near one?
2. Does it have a closet?
3. Is it connected to the main HVAC system?
If the answer to all of these questions are yes for all of the rooms that you believe are bedrooms, then you'll want to contact Gwinnett County so that they can update their records. Of course, if the construction has been modified in anyway from the original floorplan without a county permit, the county wouldn't know about the extra bedroom and would continue to send incorrect information to Trulia.
Hope that helps.