This could be caused by several things. One such cause is mites or fleas. She could have been biting the area to get relief from small mites that embed themselves much as fleas and ticks do on larger animals. If you look here you can find a treatment for your her:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=11178&N=2007+113626
If the area is raw then it must be treated soon. You can treat this much as you would on yourself or other animals with medicines like:
You want the area clean and medicated so that it will not become infected. You may need to make a small bandage out of sterile gauze and paper tape to keep your gerbil from gnawing at it even more,but watch this carefully to make sure she does not ingest any of the bandage (paper tape is much better than most other medical tapes that have more adhesive). If this does not begin to heal within a few days, I would take her in to a vet for a physical examine. Make sure to check the bandage daily to make sure she is not eating it and the wound is covered. Change as needed.
