STAB Scald is able to deal chip damage while also having a high chance to inflict foes with burn.
Earthquake gives Gastrodon another STAB option to chip specially defensive Steel types like Calm Mind Magearna, but Earth Power is also viable.
Recover lets Gastrodon stay healthy.
Toxic can be used to status foes it cannot damage or obstruct well otherwise.
Other details
Leftovers lets Gastrodon recover HP in the duration of a battle, increasing its longevity.
Storm Drain makes it able to switch safely into Water-type moves, allowing it to also force a switch on Choice Scarf Dracovish or Basculegion (as for now, since it doesn't currently learn Last Respects). The other abilities are way less useful on Gastrodon.
252 EVs on Sp. Defense are used for tanking special attacks, however a spread of 252 HP / 252 Def / 6 SpD with a Sassy nature is also viable.