Gerbils need a hidey-hole - somewhere to escape from anything that frightens them. If there's plenty of bedding in the cage - and that's an essential of gerbil keeping - they will be able to dive for cover, no problem.
Gerbils like to create little tunnels in their bedding, and will also love any other hidey holes you provide
In the wild, gerbils live together in tunnel networks. These act as hiding places, nests and chill-out zones. As long as your home setup provides a good thick layer of bedding - at least five inches deep - your pets will be content.