You keep neglecting the fact that Stealth Rock won't be up the first time Gyarados switches in. Understand this. Gyarados will take NO DAMAGE the first time he switches in, because he will switch into Mamoswine the first time he comes out. We don't need Earthquake. You want to switch your Magnezone in to Waterfall or Substitute, go for it, but understand that you did not achieve any Stealth Rock damage the first time that it happened.