How do i stop my gerbil chewing the top of it's water bottle without buying a new cage?

Any cage material that is plastic should be removed. If the cage is plastic, try your best to get a tank for the little ones. If you can't manage that, try to distract them with dyeless toilet roles and other clean paper products. Gerbils shouldn't have plastic in their environment because it can harm their insides. Good luck with your great rodents!