Good advice already on tank size/depth, but I would also advise to steer clear of bars for gerbils for the simple reason one of mine chewed the bars on a play pen when he was out playing and yanked backwards, loosening a tooth. It was hell afterwards taking him to my vets every month to have this jagged tooth trimmed and his diet no longer sustained him as he couldn`t eat normal mix and gradually lost weight and I had to watch that.
It really was a terrible time trying to manage his eating and I just want to tell anyone looking into gerbil habitats to try and stay away from any bars unless the tank has a tall vaulted roof where they can`t reach them to chew.
A large glass or Perspex tank is your safest bet. I kept a pair of boys in a 30" inch long (76cm) glass aquarium and they were fine in that. x