In my experience the best way to house gerbils is in a tank.
Generally for most small pets I recommend against a tank. I would only house gerbils, hamsters, or mice in a tank, and generally I prefer a wire/ plastic cage as it gives better ventilation.
However, gerbils are:
Escape artists and...
It does not take a gerbil long to chew through plastic. The are record fast chewers!
So it is best to have a chew-proof cage to house gerbils. Which would be either an all metal cage with no plastic (and I just don't like all metal cages very much, they stain easily and the base is never deep enough), or a glass tank/ aquarium.
Tanks are adequate for gerbils because:
bedding won't get everywhere (did I mention that gerbils are bedding flingers? They get bedding everywhere when housed in a wire cage)
escape proof (as long as you have a locking lid on top)
chew proof (gerbils can chew through plastic, but they can't chew through glass)
Gerbils seem to be quite happy in a nice large tank, and you will be happy knowing that your gerbils won't end up running free around the house, or ingesting plastic.
Now for size:
A ten gallon tank is fine for one or maybe two gerbils. A twenty gallon tank is good for two or three gerbils, you could possibly fit four though. Just make sure that they have plenty of space and don't look overcrowded.
tips and reminders:
When using a tank to house any animal make sure the lid is wire mesh so that the tank is well ventilated, never use a solid lid!