Introducing new gerbils to established ones can be tricky, but is possible. For example, if you have a gerbil living on its own it will need a new friend, as these animals can’t live happily on their own. However, if you’re trying to introduce a gerbil to an established group, then this probably isn’t a good idea.
Pleased to meet you...? Introducing gerbils to each other is sometimes a bit tricky
Introducing a gerbil to an existing group is not easy, and often doesn't work out at all. The established animals will usually look on the newcomer as an intruder and rival, and can turn nasty. In the wild, outsider gerbils appearing in a group’s burrow would not go down well, and the same applies in captivity.
Any new gerbil you bring home will therefore need its own enclosure, AND a companion of its own. They will need to be introduced into the new enclosure at the same time, and will usually settle in together.