Unfortunately there is very little chance that a gerbil of this age would accept a new companion. They are fiercely territorial unless introduced before the age of weaning. It is also a misconception to apply human feelings to your gerbil. There is very little evidence that gerbils can in fact be "lonely" – even though they may seem happier when with company they do not have the intelligence to suffer from such things as loneliness and depression. The best thing to do is give it lots of extra attention from yourself, make sure it has lots of healthy treats and plenty of toys and activities in its cage.