It is best to take your gerbil to see a veterinarian immediately if you suspect it has fractured a bone, as the injury could worsen or the bone could mend improperly, affecting your gerbil's mobility. Your veterinarian will apply a restraining bandage to the affected area to restrain movement and assist the bone in mending properly. If an open wound is present, it will need to be cleaned and suitably dressed, with topical antiseptics or antibiotics applied.
If your gerbil is in obvious pain, painkillers may be administered to help reduce the pain temporarily. Your veterinarian may also prescribe some vitamin and mineral supplements for your gerbil to help in the recovery and to strengthen the bones.
Fractures generally heal quickly in gerbils. With appropriate care, a lot of progress can be made within 7-10 days of initial treatment. You will need to place your gerbil in a small cage or enclosure to limit its movement so that healing can progress quickly. Make sure to give any oral supplements your veterinarian has prescribed.
To prevent potential limb injuries, your gerbil's caging should have solid floors, or if you do use mesh floorings, make sure that the openings are too small for your gerbil's feet to fit through them. The openings of a cage's mesh flooring are one of the most likely sources of foot and leg injuries in small animals, so this is one of the first lines of defense.
In addition, it is important to ensure that your gerbil's diet is nutritionally well balanced so that the likelihood of fractures occurring due to weakness in the bone structure -- which is closely correlated with nutritional disorders -- are reduced.