Gerbils should always be kept in same-sex pairs, as they need company. Males and females won't fight, usually, but you will soon be overrun with baby gerbils!
As long as the gerbils have been handled from an early age, they are likely to be friendly with you. Disagreements between the pair will usually be rare, and brief. The females tend to be more territorial than the males, but in a good-sized enclosure that should not be an issue.
Pairs of males may get on better than pairs of females - but most owners find that two she-gerbils get on just fine, too
Some owners say males are more docile and friendly towards each other than females. Others say they have had no problem with female gerbil pairs. However, it’s generally a good idea to choose males if you want to keep more than two gerbils together, as they seem slightly happier in larger groups than females.
There is very little size difference between male and female gerbils. Unlike some other rodents, male gerbils’ genitals aren’t very obtrusive, so males and females look very alike.