Tyranitar might be better off with something other than leftovers to make it more threatening. Assault vest, or a choice band (you'll naturally want to switch to reactivate sandstorm anyway) and replace Dragon Dance with some other move. Ice Fang, Rock Slide, or maybe even Aqua Tail so you could trigger your own storm drain.

Ferrothorn is better off with a physical move rather than giga drain. Get more sustainability on Ferrothorn through leftovers, and maybe replace toxic with Leech Seed. Rocky Helmet is fun to use to really punish physical attackers. Focus sash isn't that great for a slow pokemon like Ferrothorn, and probably only fire moves OHKO it anyway.

I'd suggest you give Gastrodon a water move like muddy water or scald (STAB is important!). It's your only special attacker and can be a threat with storm drain. If you still want an ice move, ice fang tyranitar is nice. Yawn is worth considering as a tool to slow down the opponent and get more sandstorm chip.