Your gerbil's feces should normally be firm in texture, and dark in color. A change in either texture or color, especially to a watery texture or a lighter color, usually signals a problem. Causes of diarrhea in gerbils include stresses such as changes in temperature or environment, too many vegetables or greens in the diet, and Tyzzer's disease, which is a bacterial infection caused by Clostridium piloforme
If your gerbil has diarrhea, dehydration is an immediate concern and can become serious. Try to get your gerbil to drink water. If you can trace the start of the diarrhea to a stressor such as a change in diet or new surroundings, then you can simply remove the cause to see if the diarrhea stops. Otherwise, you need to contact your veterinarian for assistance as soon as possible, but no later than within 24 hours of onset.