Gholdengo is one of the better options to deal with it because it can hit regular Garganacl with Make It Rain, and Shadow Ball against Tera Ghost Garg. On top of this, Dengo can run Covert Cloak with NP so it can just setup on Garg, even if its running Earthquake, take a hit and hit back with Make It Rain. Purifying Salt just shuts down Shadow Ball, but even more, tera water just shuts down Gholdengo.
+2 252 SpA Gholdengo Make It Rain vs. 252 HP / 252+ SpD Tera Water Garganacl: 153-181 (37.8 - 44.8%) -- guaranteed 3HKO after Leftovers recovery
And then Garg just hits it back with Earthquake for a 2HKO. Even if it's Phys Def Garg, Make It Rain is a 3HKO because of the SpA drop and leftovers recovery.

Trick from Gholdengo can cripple Garganacl, but this is so easily played around, all Garg has to do is switch out, and now Gholdengo has no way of defeating it, and is susceptible to Salt Cure damage because it isn't running Covert Cloak. So the same points from above stand.
Specs Pult can 2HKO Phys Def Garg, but all it has to do is click Recover on the first Draco escape safely. So you can imagine how much easier SpD Garg gets off. Even if its Tera Ghost Garg, Shadow Ball is still a 4HKO so, Pult doesn't do that fantastic of a job.
Short and sweet, Moonblast is a 3HKO on SpD at +1, and Focus Blast is an OHKO at +1. This all varies, is Valiant carrying FBlast? Only if its Specs. Can Moonblast 2HKO Garg? Only if it gets to +2, by this time Salt Cure has hit Valiant and done 2 turns of damage, wearing it down. Mixed CC does very solid damage, but all it has to do is Tera and it beats it.
Tera Ghost wins
Kingambit actually has a decent matchup against Garganacl compared to the others. +2 Iron Head OHKOs Regular, +2 Kowtwo Kleave OHKOs Tera Ghost, and it 2HKOs Tera Water. However, Body Press stops Kingambit, and if it wants to Tera into Flying or Fire Salt Cure is doing some damage. But it is only doing 20-24%, meaning Kingambit does win regardless one-on-one.
One of the most reliable checks to Garg, providing heavy, immediate damage regardless of its Tera. Flower Trick hits base and water, Knock Off hits Ghost. Hard hitting, super-effective, STAB damage. But, frail. Prone to Body Press, prone to Salt Cure damage. And again, suffers the same issues of Garganacl just switching to a partner that checks Meowscarada like Corviknight, Amoonguss or Kingambit.
Curse Dondozo can setup against Base Garganacl and do some solid damage, but again, tera just shuts it down on top of Salt Cure.
Tera Grass, Giga Drain Volcarona can actually beat Garganacl, Salt Cure does do some damage over time, but Garganacl just switches out.