Gerbil's love chews and gnaws, just for the fun of nibbling. But they are also a great method of allowing your pet to regulate their own tooth length. Like all rodents, gerbils' teeth grow throughout their lives, only worn down by the hard foods they eat. However, sometimes pet gerbils don’t get enough sufficiently tough foods to wear down their teeth naturally. That's why they like having something in their enclosure to gnaw.
It's good to supply a chew or gnaw toy for the gerbils to satisfy this instinct with, rather than letting them chew their fixtures and fittings and cage bars.
Choose chews carefully, and go for gnaw-good rather than no-good!
There are lots of chews and gnaws available in pet stores. If you want to make one yourself, make sure you don't use treated wood - i.e. anything with varnishes, stains, pesticides, or other chemicals on or in it. It’s also best to be sure that it’s a type of wood that doesn’t splinter too easily (e.g. cedar), as this can risk choking, and also presents a hazard in the form of breathable wood dust.