Fruit tree prunings are ideal. Pine cones are a great hit too. This isn't really a good idea. Whilst gerbils are not afraid of large predators like cats and dogs, these animals can easily injure or kill a gerbil. Even contact between different pets of a similar size can be a major mistake.
Gerbils tend to be extremely aggressive towards other small rodents.
They will normally attack and attempt to kill hamsters, rats and mice. Before gerbils are about 8 weeks they will normally except the company of other Gerbils. Unfortunately, after that age it gets more difficult to introduce them. Males are much more easy to introduce to one another. They will normally accept a Gerbil that is under 8 weeks old and be very protective towards it.