I got a female gerbil not sure how old she is. I want another playmate for her. Is it safe to get another female from a different litter and let them be in the same cage?

I'm sure Bubbles will love a little friend to play with (I have two of my own but they were from the same litter) you must be careful because sometimes this doesn't work. I would buy a seperate cage and keep the new gerbil in the new cage, every other day swap the gerbils around so that the new gerbil will be in the old cage and the old gerbil in the new one so they get used to one anothers smell. Keep doing this until you are confident they are used to eachothers smell and let them into one cage only. Stay with them and watch them for a while until they get used to eachother and are happy friends! Enjoy.
Tip: To speed the prosess along abit put the two cages next to eachother so that they can see eachother and will get used to their appearence as well as smell!

females are more peaceable together than males, also, try dividing the cage with wire mesh and putting one gerbil one one side and the other on the other side. the gerbils will hopefully get used to each other in a while. also, the gerbils will see each other, and smell each other through the wire mesh. good luck!(I have 2 gerbils myself)