However, if they can't, then there is a problem. The odd scab, especially around the base of the tail, probably indicates fighting has broken out. This may have been a trivial argument over an extra-large sunflower seed. But it may also be a warning of worse to come. You need to monitor the situation carefully.
External parasites are rare on gerbils.
However, if your gerbil has an inflamed, scabby, bald round patch on its coat, then it could just possibly be ringworm. This is highly contagious. Take no chances and consult your vet. It comes as something of a shock when your gerbil suddenly emerges minus half of its tail. The gerbil's tail with its striking black tip, is designed so it is easily shed if caught by a predator.