Gastrodon can do one thing and one thing alone in Ubers: wall the most popular Pokemon in the metagame, Kyogre. With access to Recover, Toxic, Mirror Coat, Yawn, and other annoying moves, Gastrodon can come in for free on Kyogre and provide team support, spread status, or just be annoying in general. Whilst Gastrodon faces competition from Shedinja and Quagsire in the Kyogre-annoyer field, it requires less team support than Shedinja to be successful, and has far superior special bulk to Quagsire, arguably making it the best option. Not only that, but Gastrodon can also hit surprisingly hard if it manages to get a Storm Drain boost, meaning it isn't complete Taunt bait.
One of Gastrodon's best qualities is its ability to come in on offensive Kyogre variants and stop them in their tracks. With immunities to both Water Spout and Thunder, and taking only 22.1% - 26.4% from an unboosted Ice Beam, Gastrodon can simply hit Kyogre with Toxic and spam Recover until Kyogre dies or switches out. Needless to say, this set is most effective against Kyogre's Choice sets, but it can also deal with other sets in a pinch. Support sets that run Thunder Wave over Water Spout are easily dealt with, and offensive Calm Mind sets also run into trouble. Even a +6 Ice Beam isn't guaranteed an OHKO, at which point Toxic will likely have killed Kyogre. RestTalk sets are troublesome as they can PP stall Gastrodon successfully, although if Sleep Talk selects Surf persistently, then Gastrodon can bolster the power of its own Scald to devastating levels, steamrolling Kyogre along with the rain boost.
Scald is Gastrodon's primary attacking move, as the Special Attack boost gained from switching into Kyogre's Water-type moves allows it to hit neutral targets hard. It also has the advantage of occasionally burning Steel-types, who can otherwise come in on Gastrodon fairly easily. Recover is godly for Gastrodon, and vastly increasing its stalling power against many special attackers. Toxic is used to stack up residual damage and help Gastrodon stall in the first place, although if Toxic Spikes support is provided then Gastrodon can afford to run an extra attacking or utility move in its place. Mirror Coat is the primary option for the last slot because it allows Gastrodon to check other special attackers, such as Palkia and Dialga. With the given EV set, Gastrodon is never 2HKOed by Timid Lustrous Orb Palkia's Spacial Rend, and Gastrodon can thus rebound the attack with Mirror Coat for an easy OHKO on Palkia, recovering the damage off at some other point. Dialga's Draco Meteor also fails to 2HKO Gastrodon, letting it switch in and recover off the damage. However, Gastrodon has several other viable options for the last slot. Earth Power provides a useful secondary STAB and can hit Dragon-types harder than Scald after the Storm Drain boost. Clear Smog can be used to deal with Calm Mind users more directly; sadly, it fails against Steel Arceus. On the other hand, Mirror Coat is arguably more effective against some Calm Mind users, as they will often fail to OHKO Gastrodon with their attacks, allowing it to bounce back the damage for an easy OHKO of your own. For example, a +6 Steel Arceus Judgment only deals 57.7% - 68.3% to Gastrodon.Team Options & Additional Comments >>>
Yawn lets Gastrodon try to phaze or induce sleep, but fails against the somewhat common RestTalk Kyogre, and synergizes poorly with Toxic's and Scald's chances to inflict status. Memento is an interesting option, or at least it would be if Gastrodon was fast enough to use it well. Counter can also be used somewhat effectively; Scarf Zekrom's Outrage will always fail to OHKO, but don't ask me how the poor sea slug would switch in on it. The mixed set's Dragon Claw will also fail to OHKO, but its Outrage stands a 50% chance, so be warned. Defensive investment is highly required in this case so that Gastrodon can survive all kinds of Life Orb-boosted Outrages; Gastrodon deals fairly well with Kyogre even without Special Defense investment. Stockpile is also a viable option, but inevitably leaves Gastrodon setup bait to the multiple Calm Mind users in the tier, and other boosters such as Extreme Killer Arceus.
Gastrodon also gets Sand Force via Dream World. Though in itself an excellent ability, it is rather out of place on Gastrodon because of its poor offenses, and the only reason one would use Gastrodon in Ubers would be to tackle Kyogre. Furthermore, sandstorm is not a prevalent weather effect in Ubers, and thus this ability sees little use on Gastrodon.
Checks and Counters
In one-and-a-half words, Grass-types. Shaymin-S completely destroys Gastrodon no matter what it attempts to do, easily setting up Substitute if Gastrodon lacks Ice Beam. Ferrothorn can OHKO all Gastrodon variants with an uninvested Power Whip, or set up Spikes and Stealth Rock with impunity. Grass Arceus poses an enormous threat as it can force Gastrodon out for a free boost. Even if Gastrodon stays in and uses Toxic, Arceus can often get multiple boosts at no real cost if it packs Refresh. Groudon can hit Gastrodon hard and set up on it, although if Groudon is present on the opponent's team, Kyogre will most likely not be there, giving your team a small advantage. Some variants of Kyogre can also prove annoying to face, namely RestTalk and Substitute + Calm Mind: the former easily PP stalls Gastrodon, while the latter blocks status effectively.
Generally speaking, however, Gastrodon just isn't effective in the tier at all outside of its niche of dealing with certain special attackers. If it is being used in a situation where it isn't reacting to a special attacker, chances are it isn't going to pose a threat to your team.