I actually kept gerbils once my first one was a female who was housed on her own until a year old, she got lonely and started thumping her back feet on the glass platform of her tank. I was at college at the time studying animal care, my teacher said that's a sign of loneliness. Unless you get signs of behavior such as feet thumping to signal its lonely, then I would suggest you let them live a single life. Its easier to pair an older gerbil with a younger one of the same sex or opposite sex but expect lots of babies, and introductions should be done on neutral ground. There is lots of infos in books that help you to introduce a new one, always be prepared they might not get on at all and might need to live alone.
With my female she was too old to breed and my local pet shop only had in a litter of males, I took the one who was being beaten up by his brothers. His nose was bloody, he was unhappy, and obviously needed to get away from his horrible litter mates. I introduced the two together in an empty cage, my female called Misty instantly pounced him and licked his nose. The whole time he squeaked and squeaked even tried to fend her off, but she punched him then went right back to licking his nose and caring for him. I called him Toffee the two lived together until I had to have Misty put to sleep due to severe illness, Toffee got depressed right after she never came home and he refused to do anything but lay in the bottom of the tank. He didn't eat, he didn't drink, he just laid there.
I would have gotten him a male playmate to cheer him up but as is a strange fate, pet shop only had females in this time around and I picked up a little black one I called Popcorn. The instant she was put in his tank he perked up and began to cuddle her, they had many babies of which I kept their last litter they had that was two black males. One of them had a front hand that he couldn't use at all it remained curled up, but it never bothered him at all.
