and Hammer Arm...do I really need this one? It's already holding Iron Ball, so it's slow enough. Shouldn't I switch it with Earthquake?
Yes you should and why do you have both Gyro ball and Bullet punch it seems pretty wierd and pointless unless you have it to defeat pokemon like Weavile.
I'd say to replace Bullet Punch with Ice Punch for its Ground type weakness. Also since your holding an Iron Ball, replace Hammer Arm with Earthquake as you said because its slow enough and for its Fire weakness.
Hope this helps..
If you realy want to you could replace one of the moves with Agility to offset the use of Hammer Arm.
I agree with the above: get rid of bullet punch and replace it with ice punch. Also, replace gyro ball with meteor mash and get rid of the Iron Ball, metagross is slow enough without it.