Gerbil pups are generally given everything they need from their parents--both of them! Believe it or not, gerbils are wonderful parents! If the mother does not seem to be tending to the babies as often as you think she should, cover one half of the tank with a towel and shine a light on one corner of the cage for the babies (make sure not to put it too close so it does not get too warm). If the babies are still less than a few days old, you can look at their stomach and see an off-white spot beneath the skin that looks like a bean; these are called milk beans, and are actually the milk in their tummies. As long as these milk beans are there, mom is feeding them fine. If the gerbil pups do not have these beans for a few hours, you may have to buy kitten replacement milk and feed the pups through an eyedropper. If you can help it, try leaving the parenting to mom and dad. :]
Hope this helps!