2020 updates by Cathy Bickel
A male and female gerbil left together will mate. A female in heat will signal her receptiveness. The mating ritual usually takes place in the evening. The male will mount the female from behind; if she is receptive, she will raise her hindquarters to assist him. After a few seconds, each will check their undersides. Then the male will chase the female around and repeat the performance. This will go on for several hours.
If the mating is successful, about 24 days later, the female will give birth to a litter of pups. If she is already nursing pups, the gestation period for the second litter may be as long as 45 days. The male and female will mate again within a few hours of the birth. Do not remove the male right after the birth. Gerbil fathers are nurturing and helpful, and the mother may become distressed and neglect her pups if he is removed. Yes, you will be having a second litter. See Separating Gerbils and Pup Development.