Gerbils are so cute and lots of fun.
I had several pairs of them when I was a kid and just recently had a pair of them for my kids.
Here are some links that should help answer your questions.

Gerbils as pets

abc gerbils

pet gerbils

Caring for gerbils

I think the last link should answer your questions.
A pair of gerbils will do fine in a 10 to 20 gallon tank.
The last link seems to have the most info, you can also look at the color chart that shows all the wonderful colors they come in.
Let me know if you have any more questions about gerbils.