Gerbils will need a place to nest as well as hide. Remember what ever you place in their cage will be chewed and if ingested cause intestinal blockage or punctures. A wooden hut or ceramic pots work great as well as bendy bridges (shown above).
Gerbils need adequate chewing materials to keep their teeth from over growing and causing health issues. Local pet stores as well as online stores sell a variety of items. Constantly purchasing chew items can become quite expensive. You can supply them with a lots of cardboard from around the house such as toilet paper and paper towel rolls, tissue boxes, non wax food boxes and so on. Do not give your gerbil corrugated cardboard as the glue used can be toxic. It is best to keep a stock of willow or apple sticks from the pet store in their tank for when they desire something harder to chew. As always check your gerbils teeth at least weekly.