Tbh, I'm not too sure about it myself. Gyarados doesn't have much to work with.Kinda iffy on this one, as fire blast gyarados just doesn't sound right to me. Sure, it can lure in ferrothorn, but +1 bounce already does a huge amount of damage. Holding back on this for now.
However, I will say that since this a wallbreaking set rather than a Dragon Dance set, it's a better lure than Bounce. SubDD w/ Bounce is vulnerable to either being stalled out with Protect or being paralyzed by Ferrothorn, so it's really not the best solution. Life Orb Fire Blast on a wallbreaker set nearly eliminates Ferrothorn and Gyarados is never pressured to stay in because it doesn't boost on the wallbreaker set. Toxic also allows it to get past some of the non-Steel or Poison type Pokemon that Gyarados will struggle to beat like Slowbro and Unaware Clefable.
I'm not sure if this is enough to include the set, but I think it needs to be looked at from another perspective.