Gerbils don't need very many store-bought toys. In fact, I rarely go to the pet store, as I build most of my gerbils' toys myself. Plastic toys aren't good as your gerbils will chew though them quickly and the plastic shreds are potentially dangerous.
The best toy for gerbil is--believe it or not--cardboard! Toilet paper tubes are the most popular with gerbs, but cereal boxes, cracker boxes, and any other kind of non-waxy cardboard.
Hay is much appreciated too, if you can get it. Normal pet store Timothy hay is great, but very expensive
Huts
Some kind of hut is a must for gerbils. Huts and other toys provide lots of burrowing support, and a nice wooden one is great for sleeping.
