Heatran, even with HP [Grass], isn't going to be dealing any significant damage to this guy. And without Toxic to threaten it, Gastrodon has even less to fear. It might not be landing anything resembling a OHKO (2HKO with Scald and SR damage at best), but Gastrodon will counter Heatran up and down.
Articuno @ Leftovers
248 HP / 8 Def / 252 SpD
- Ice Beam
- Heal Bell
Articuno can avoid EQ if it stalls out Gastrodon's attacks with Pressure, it can Toxic Gastrodon so it dies slowly, and can heal itself from getting Burned or Poisoned, and is barely hurt by Scald and Ice Beam thanks to its high SDef, all without really needing to make use of Roost, which'd make it vulnerable to EQ.