We had a gerbil that chewed out of it's cage once ... and sadly, it got behind the refrigerator and was killed by the fan.

If your gerbil is chewing at the ends of the tubes, you can put a piece of wire or even a straightened paper clip around the end of the tube. The don't like to chew metal, I guess.

I ended up putting our gerbils in a wire cage, so they wouldn't chew through the plastic anymore.