Wheels are a great tool for enrichment. Gerbils love to run. Wheels need to be a minimum of 8" in diameter. Larger wheels are preferred. As a show breeder its important for gerbils to have a straight tail. Some gerbils will develop wheel tail from running on wheels. They lift their tails and arch them over their back to keep their tail off the wheel. Using very large wheel of 11 or 12" in diameter can also prevent wheel tail. This can put a serious curve into how your gerbils hold their tails. I prefer to use saucers with my gerbils. Saucers do not cause wheel tail.
When choosing a wheel I prefer to use metal wheels, as they are chew proof. I tend to avoid most plastic wheels as these will get destroyed. The biggest factor in choosing a wheel is how difficult it is to clean. Gerbils pee and poop while running, and this can build up on the wheels running surface. Wheels need to be cleaned regularly as they will begin to stink rather acutely with use.