Unfortunately, even with Metang's low speed, he's still too fast to use Gyro Ball effectively. Even with an absolute minimum speed stat, you'd have to be facing an opponent with the speed of a Timid Zebstrika just to match the power of Meteor Mash. Of course, it would be much more worth it if you were using an Iron Ball (which would be pretty cool with Trick), but the loss of the Eviolite just isn't worth it.