Why Gastro Sucks: An Essay
1. Poor bulk in relation to power creep
Gastro must invest fully into either def or spdef to check anything at all. Mixed def sets are essentially irrelevant as they fail to check/counter the things gastro really wants to switchin to, like MegaPert or AshNinja. For instance, MegaPert 2hkos mixdef gastro w EQ and LO Tomo 2hkos w hurricane.
252+ Atk Swampert-Mega Earthquake vs. 204 HP / 116 Def Gastrodon: 235-277 (56.7 - 66.9%) -- guaranteed 2HKO
252+ SpA Life Orb Tomohawk Hurricane vs. 204 HP / 188+ SpD Gastrodon: 207-243 (50 - 58.6%) -- guaranteed 2HKO
So, this essay will focus solely on entirely physdef and entirely spdef versions of gastro, both of which are fairly mediocre in general. Being forced to run fully invested in one stat or the other really limits Gastro as its main goal is to be a bulky blanket check for BO/balance. In this current generation, its ability to be a mixed wall suffers gretly due to power creep, not that it was particularly great in ORAS either. Physdef versions, while doing a decent job of checking threats like Mega Swampert and Zygarde, still lose to Lando T (SD adamant has a decent chance 2hko w EQ after rocks, +2ZFly/earth plate EQ both OHKO from full), Mega Medicham, Mega Mawile, and most other wallbreakers in the tier, even excluding those with a type advantage.
+2 252+ Atk Landorus-Therian Earthquake vs. 252 HP / 252+ Def Gastrodon: 357-420 (83.8 - 98.5%) -- 31.3% chance to OHKO after Stealth Rock
252+ Atk Pure Power Medicham-Mega High Jump Kick vs. 252 HP / 252+ Def Gastrodon: 355-418 (83.3 - 98.1%) -- 31.3% chance to OHKO after Stealth Rock
252+ Atk Huge Power Mawile-Mega Play Rough vs. 252 HP / 252+ Def Gastrodon: 255-300 (59.8 - 70.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ Atk Guts Colossoil Earthquake vs. 252 HP / 252+ Def Gastrodon: 235-277 (55.1 - 65%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Life Orb Garchomp Earthquake vs. 252 HP / 252+ Def Gastrodon: 387-458 (90.8 - 107.5%) -- 81.3% chance to OHKO after Stealth Rock
Basically, any decently powerful attacker can 2HKO/OHKO after a boost. Gastro just dies to everything.
SpDef fairs a little bit better, but it still underwhelms in general. It cannot check boosted attackers at all and its only way to slow them down is clear smog, which is a necessity this gen as otherwise it can only toxic them. Quiver dancers like volc+aurumoth run straight through spdef gastro (giga+psyshock, respectively). Tail Glowers like manaphy and syclant can boost up and annihilate Gastro; admittedly, mana does need to be running energy ball, but gastro is still a shaky check. Basically; it cannot wall anything that can boost and barely survives vs strong, unboosted special attackers. Even scarf lele has a chance to 2hko. It cannot check anything except elecs reliably, as almost all relevant fire types commonly carry ways around it (solar beam char-y, giga volc, pyroak in general, grassium heatran, etc). This forces you to run a second fire check which makes teambuilding that much harder. Even weaker mons like kerfluffle can possibly 2hko gastro.
+3 252 SpA Life Orb Syclant Ice Beam vs. 252 HP / 252+ SpD Gastrodon: 355-419 (83.3 - 98.3%) -- 31.3% chance to OHKO after Stealth Rock
252 SpA Tapu Lele Psychic vs. 252 HP / 252+ SpD Gastrodon in Psychic Terrain: 180-213 (42.2 - 50%) -- 0.4% chance to 2HKO after Stealth Rock and Leftovers recovery
252 SpA Life Orb Kerfluffle Focus Blast vs. 252 HP / 252+ SpD Gastrodon: 191-226 (44.8 - 53%) -- 27.3% chance to 2HKO after Stealth Rock and Leftovers recovery
In summary: Gastro doesnt wall much and it fails to be compress roles effectively.
2. Susceptibility to hazards, status, and knock off
Because Gastro only barely survives most attacks, any chip damage/lack of leftovers it encounters throughout the match means it becomes essentially useless as it is no longer an effective switchin. So, if gastro wants to switchin to something like Rotom to eat a hydro/volt switch, but it gets burned instead, it turns out to no longer be able to wall MegaPert, for example (assuming physdef). Toxicing gastro has an even worse effect for obvious reasons. Spikes, better in SM than in ORAS, have a similar effect, as gastro suddenly cannot switchin to many threats even if fully healthy. Please make note of the wall of calcs to follow:
252+ Atk Swampert-Mega Earthquake vs. 252 HP / 252+ Def Gastrodon: 183-216 (42.9 - 50.7%) -- 46.5% chance to 2HKO after Stealth Rock, Leftovers recovery, and burn damage
252+ SpA Life Orb Tomohawk Hurricane vs. 252 HP / 252+ SpD Gastrodon: 192-227 (45 - 53.2%) -- 91.4% chance to 2HKO after 1 layer of Spikes and Leftovers recovery
+2 252+ Atk Dread Plate Bisharp Knock Off (97.5 BP) vs. 252 HP / 252+ Def Gastrodon: 373-439 (87.5 - 103%) -- guaranteed OHKO after 1 layer of Spikes
252 SpA Choice Specs Greninja-Ash Dark Pulse vs. 252 HP / 252+ SpD Gastrodon: 181-214 (42.4 - 50.2%) -- 83.2% chance to 2HKO after 2 layers of Spikes and Leftovers recovery
252 SpA Life Orb Tornadus-Therian Hurricane vs. 252 HP / 252+ SpD Gastrodon: 169-200 (39.6 - 46.9%) -- 96.1% chance to 2HKO after 3 layers of Spikes and Leftovers recovery
The list goes on and on and on. Basically, Gastro loses to the few offensive mons it can actually check if statused/chipped in any way. Its bulk does not allow for any imperfect circumstances, which are virtually impossible to prevent.
3. Utter passivity
Gastro cannot kill anything ever. Spdef sets dont have room for anything but stabs/clear smog/recover which leave it walled by so much. Physdef sets can either run ice beam (brought up by ppl arguing for gastro rise but its shit dont use it) or toxic, which does what it always does. Basically, so much stuff can come in for free and do as it ishes vs Gastro. It serves as a momentum sinkhole on teams that dont rly want that. Offensively, its outclassed by everything from MegaPert to Seismotoad. Defensively, it struggles heavily in this meta.
0 SpA Gastrodon Ice Beam vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 88-104 (21.7 - 25.7%) -- 0.9% chance to 4HKO
0 SpA Gastrodon Scald vs. 0 HP / 0 SpD Landorus-Therian: 194-230 (60.8 - 72.1%) -- guaranteed 2HKO after Stealth Rock
0 SpA Gastrodon Ice Beam vs. 252 HP / 24 SpD Landorus-Therian: 284-336 (74.3 - 87.9%) -- guaranteed 2HKO
Gastro should drop to D if its gonna move at all. It is heavily outclassed, fails to do what it wants, lacks any utility options (ie SR), and just generally sucks.