When selecting a gerbil, first of all look at all the gerbils in the cage and the conditions they are kept in. They should be in good healthy condition and not be kept in overcrowded cages. The cages themselves should be clean and the gerbils should have access to food and water. The gerbils should be separated by sex with males in one cage and females in another. If buying from a pet shop and the shop staff cannot guarantee the sex of any gerbil this is not a good sign and buying could mean buying a pregnant female - and problems.
The gerbils should ideally be 5-6 weeks of age - gerbils should not be sold younger than this.
Once you have found a good pet shop or breeder inspect the gerbils carefully. If one gerbil appears sick the others may also be at risk of being ill so it is best to look in another cage or another shop.
Check that the gerbil is bright eyed and alert when it is awake. It should be inquisitive and not too nervous. Beware of any gerbil that has runny or sticky eyes, runny nose, sneezing, wet or dirty bottom, matted fur, seems lethargic or does not have a firm body. All of these things can indicate a sick gerbil.
Once you have seen a healthy gerbil that you like, ask if you can handle it so that you can see how tame it is. If a shop does not allow you to handle the gerbil you have no opportunity to assess how suitable it is as a pet and cannot check its health properly before buying so is best avoided.