Gerbils should always be kept in pairs, so if you have one gerbil, you’ll want to get another of the same gender so that you’re not overrun with babies!
Whether you adopt a pair of female gerbils or a pair of male gerbils is up to you, and although there are trends in behaviour between the genders, they are just that - trends. There are no guarantees that your gerbils will follow them, and so by and large the decision is down to personal preference. Gerbils that have all their needs satisfied and are handled from a good age are likely to be friendly.
Some owners suggest that pairs of males may get on a little better than pairs of females
Some, but not all, owners have said that males are more docile and friendly towards each other than females are. It’s generally considered to be a good idea to choose male gerbils if you want to keep more than two together at any one time - they seem slightly more accepting of one another than groups of females.
In terms of looks, there is very little size difference between the genders. Unlike some other rodents, male gerbils’ genitals aren’t very obtrusive, so males and females look very alike.