It is not difficult to answer as I will demonstrate. Gerbils are in fact extremely easy to sex from about 5 weeks old upwards. With a little experience you should be able to reliably sex gerbils from the third week or sooner. There are four main differences between male and female gerbils. Adult males are significantly larger, although this is not a precise method of sexing because gerbil size can depend on a number of other factors.
Adult males have a large bulge at the base of the tail which is their scrotum.
This is totally absent from females. This bulge is obvious by 6 weeks of age and can be seen at least two weeks earlier. Female gerbils have more obvious nipples. Finally and most importantly, the distance between the urinary and anal openings in female gerbils is much smaller than it is in males. This method can be used reliably from at least three weeks of age.