It may not beat gambit and heatran, but everything else gets destroyed by stored power+ dazzling gleam. Ghold which resists it's stab will eventually be defeated by enough boosts. Reminder that it gets more powerful each turn just by existing.
Some calcs:
(long list of calcs cut for brevity)
I think it's pretty telling that even at the worst case scenario where you've let Espathra set up 2 calm minds it STILL doesn't OHKO any of our steel types. And almost everything 2HKOs it back:
252 Atk Excadrill Earthquake vs. 252 HP / 0 Def Espathra: 255-300 (64.7 - 76.1%) -- guaranteed 2HKO
4 Atk Iron Treads Knock Off (97.5 BP) vs. 252 HP / 0 Def Espathra: 234-276 (59.3 - 70%) -- guaranteed 2HKO
252 Atk Iron Boulder Mighty Cleave vs. 252 HP / 0 Def Espathra: 222-262 (56.3 - 66.4%) -- guaranteed 2HKO
4 Atk Life Orb Deoxys-Speed Knock Off (97.5 BP) vs. 252 HP / 0 Def Espathra: 265-312 (67.2 - 79.1%) -- guaranteed 2HKO
0 SpA Heatran Magma Storm vs. +2 252 HP / 0 SpD Espathra: 102-121 (25.8 - 30.7%) -- guaranteed 3HKO after trapping damage (to match your own 3HKO)
252 SpA Gholdengo Shadow Ball vs. +2 252 HP / 0 SpD Espathra: 204-240 (51.7 - 60.9%) -- guaranteed 2HKO
252+ Atk Black Glasses Kingambit Sucker Punch vs. 252 HP / 0 Def Espathra: 470-554 (119.2 - 140.6%) -- guaranteed OHKO (also this is with no allies fainted lmao)
plus you have emergency checks in skarm and ting-lu that can ww it out
+2 252+ SpA Espathra Stored Power (140 BP) vs. 252 HP / 0 SpD Skarmory: 282-333 (84.4 - 99.7%) -- guaranteed 2HKO
+2 252+ SpA Espathra Dazzling Gleam vs. 252 HP / 252+ SpD Vessel of Ruin Ting-Lu: 202-238 (39.2 - 46.3%) -- guaranteed 3HKO after Leftovers recovery
And I feel the need to reiterate,
you have massively misplayed if you are in this situation. You let espathra get in and set up 2 calm minds for free without any damage on it, and you still have ways out of the situation, particularly in top metagame threat Gambit. And don't even talk to me about aurora veil or screens. Can you not see how the pokemon whose whole gimmick is gaining power by surviving more turns has negative synergy with limited screen turns? In your proposed scenario you've already used half of your screen turns simply setting up, which when you do inevitably fall leaves you needing to re-set up screens.
I really can't see espathra without tera being scary at all. If anything it's a worse version of several calm mind stored power sweepers we have now, like Lati@s or Iron Crown, since they actually have all of coverage, bulk, and power. If you let them get 2 calm minds up, chances are the game is over already, which is a far cry from espathra. Without tera, I'd even wager it wouldn't even be an OU pokemon.