ye it would just be broken , adv doesn't have a recover using water type that also resists rock, we have miltoic and suicune and wish tect vaporeon who all fear rock slide despite not being 2hkoed by it by virtue of CB, DD and also just raw 30% flinches, all the while they also take sand chip, if you attack as they swap you arent healing at all, same with if u get flinched. As a result, when you run one of theses you are still forced to run a true resist with them - or you get flinched once by CB tar, DDtar or CB aero and its over, this can come in many forms but the crux of the matter is, if you switch one of those waters into a faster, boosted rock attack it is really gonna struggle to convincingly win the 1v1. gastrodon would entirely solve that issue, sure you still take a shit tonne from CB tar focus punch but like - everything does, even swampert and milo.
To drive the point home lets do some calcs - lets calc the 2 most similar pokemon (milotic and swampert) and then gastrodon against important attacks so we can clearly see the comparison.
+1 ttar rockslide
252+ Atk Choice Band Tyranitar Rock Slide vs. 252 HP / 252+ Def Gastrodon: 94-111 (22 - 26%) -- possible 5HKO after Leftovers recovery
252+ Atk Choice Band Tyranitar Rock Slide vs. 248 HP / 216+ Def Swampert: 81-96 (20 - 23.8%) -- possible 6HKO after Leftovers recovery
252+ Atk Choice Band Tyranitar Rock Slide vs. 252 HP / 252+ Def Milotic: 173-204 (43.9 - 51.7%) -- 10.9% chance to 2HKO after sandstorm damage and Leftovers recovery
Clearly with recover and rock resistance and sand immune gastro is taking this the best.
+1 ttar focus punch
252+ Atk Choice Band Tyranitar Focus Punch vs. 252 HP / 252+ Def Gastrodon: 251-296 (58.9 - 69.4%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Tyranitar Focus Punch vs. 248 HP / 216+ Def Swampert: 218-257 (54 - 63.7%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Tyranitar Focus Punch vs. 252 HP / 252+ Def Milotic: 230-271 (58.3 - 68.7%) -- guaranteed 2HKO after sandstorm damage and Leftovers recovery
We can see that when we put them all on an even playing field, swampert shows us why it can somehow take CB focus to the face and get back in the game thanks to lefties and tect, gastro takes about 6% more than pert does and milotic takes 5% more, once we factor in the sand immune, swampert and gastro are actually pretty ahead of milo, especially when we start to consider how sometimes cheeky CB tar will focus punch again to nab some extra damage on a milo or bait an attack rather than a recover, gastrodon is more resilient to this kind of play as it has passive recovery it is punished less for each incorrect predict. Non CB tar using focus punch is fully recoverable by gastro and milo, gastro however will also be able to nulify some additional potential spikes damage thanks to its lefties.
+1 aero rockslide
252 Atk Choice Band Aerodactyl Rock Slide vs. 252 HP / 252+ Def Gastrodon: 72-86 (16.9 - 20.1%) -- possible 7HKO after Leftovers recovery
252 Atk Choice Band Aerodactyl Rock Slide vs. 248 HP / 216+ Def Swampert: 63-75 (15.6 - 18.6%) -- possible 8HKO after Leftovers recovery
252 Atk Choice Band Aerodactyl Rock Slide vs. 252 HP / 252+ Def Milotic: 133-157 (33.7 - 39.8%) -- 29.5% chance to 3HKO after Leftovers recovery
+1 aero double edge
252 Atk Choice Band Aerodactyl Double-Edge vs. 252 HP / 252+ Def Gastrodon: 154-182 (36.1 - 42.7%) -- 96% chance to 3HKO after Leftovers recovery
252 Atk Choice Band Aerodactyl Double-Edge vs. 248 HP / 216+ Def Swampert: 134-158 (33.2 - 39.2%) -- 11.7% chance to 3HKO after Leftovers recovery
252 Atk Choice Band Aerodactyl Double-Edge vs. 252 HP / 252+ Def Milotic: 141-167 (35.7 - 42.3%) -- 93% chance to 3HKO after Leftovers recovery
As you can see, aero cant even break gastrodon without 3 layers up,and even then it has to click dedge, milo and pert struggle much more tho with either edge or slide plus spikes are very punishing!
Lastly lets make a note of its utility vs offmie, even if storm drain doesn't work the same in adv - we can only theorise ofc, but having recover means you can easily recover off 8pp of hydro pumps when they don't even 2hko without spikes up, much better than milo which has to avoid a tbolt crit or pert who while it can check offmie, cannot switch directly in and also can find itself checked by offmie at times due to lack of reliable recovery.
252 SpA Starmie Hydro Pump vs. 252 HP / 44 SpD Gastrodon: 182-216 (42.7 - 50.7%) -- guaranteed 3HKO after Leftovers recovery
Shortcomings
lack of refresh!! - that's a BIG one, much more susceptible to toxic,
No torrent - this makes u worse at beating ddtar in the lategame.
lower attack than pert, you are gonna have to make do with a weak EQ
speed - naturally slower than everything of note bar lax and lix
Offensive viability - you cant threaten to be offensive focus punch or sub endeavor or even simply tech on focus punch to a defensive set to help keep tempo on frail offense teams, and this is a shame for a non preview gen, you are revealing much more info when it hits the field than a pert would.
Conclusion
Gastro would be a pretty nice addition onto balanced and "do nothing style builds" possibly as compression in the milo + metagros, milo+claydol+dug style slots, and also in the place of swampert potentially on balanced spikes style builds, however on teams where the tempo is faster they may benefit from swamperts reliable checking and ability to reduce the number of free turns by attacking straight on the switch and using protect for recovery instead, you probably partner up with mons like celebi to make a nice beefy defensive backbone. It would certainly be OU by sheer virtue of compression imho, honestly you could just use it like a swampert bliss slow pace stall, and replace wish on ur bliss with heal bell :)