There's no analysis right now for Grookey, so we can't tell its counters from that. All the recommendations are based on what higher ladder >1700 around uses.
Mentioning the most frequent counters based on Grookey's item usage by usage stats:
| Checks and Counters |
| Foongus 79.276 (83.30±1.01) |
| (29.6% KOed / 53.7% switched out)|
| Salandit 79.017 (90.48±2.86) |
| (29.5% KOed / 61.0% switched out)|
| Koffing 77.182 (83.01±1.46) |
| (28.1% KOed / 54.9% switched out)|
| Woobat 74.851 (82.10±1.81) |
| (34.7% KOed / 47.4% switched out)|
| Trubbish 71.008 (85.42±3.60) |
| (17.7% KOed / 67.7% switched out)|
| Oddish 70.442 (85.71±3.82) |
| (23.8% KOed / 61.9% switched out)|
| Skorupi 70.311 (89.74±4.86) |
| (28.2% KOed / 61.5% switched out)|
| Stunky 68.080 (85.71±4.41) |
| (30.2% KOed / 55.6% switched out)|
| Rowlet 67.034 (89.66±5.66) |
| (24.1% KOed / 65.5% switched out)|
| Ponyta 66.878 (74.40±1.88) |
| (27.1% KOed / 47.3% switched out)|
| Bulbasaur 61.757 (76.52±3.69) |
| (26.5% KOed / 50.0% switched out)|
| Vullaby 61.537 (65.43±0.97) |
| (30.8% KOed / 34.7% switched out)|
Source (Grookey's check and counter section)
Among these, Foongus is most used for countering Grookey. Vullaby has the least, so I don't agree with PrimalKyogre on that point. The EV spread on Vullaby is never Maximum Defense, so using Vullaby isn't the best choice.
| Spreads |
| Jolly:0/236/76/0/0/196 31.835% |
| Modest:0/0/76/236/0/196 16.627% |
| Adamant:0/236/76/0/0/196 7.984% |
| Careful:116/0/76/0/156/116 4.491% |
| Modest:0/0/156/236/0/116 4.213% |
| Adamant:0/236/0/76/0/196 3.444%
Source: Vullaby's used spreads in Usage stats
Most used EV spread sets are offensive. Therefore, do not use Vullaby as an Grookey counter. Plus, Vullaby's a better U-Turn pivot with Berry Juice (47% and 67% respectively) compared to Eviolite and Roost (27% and 14% respectively). That makes Vullaby not a good choice to send to Grookey. So, what does ladder use the most for it?
Foongus @ Eviolite
EVs: 124 HP / 156 Def / 156 SpD
- Giga Drain
- Sludge Bomb
Foongus is honestly the best counter. In high ladder, Grookey carries Life Orb 71%, Evilolite 19% and Grassy Seed a mere 5%. Acrobatics isn't even listed in the commonly used moves, therefore it's your best bet for handling Grookey.
236 Atk Life Orb Grookey Wood Hammer vs. 124 HP / 156+ Def Eviolite Foongus in Grassy Terrain: 4-5 (16 - 20%) -- possible 6HKO after Grassy Terrain recovery
236 Atk Grookey Acrobatics (55 BP) vs. 124 HP / 156+ Def Eviolite Foongus: 8-10 (32 - 40%) -- 1.1% chance to 3HKO after Grassy Terrain recovery
On the other hand, Sludge Bomb is a nice OHKO.
0 SpA Foongus Sludge Bomb vs. 0 HP / 0 SpD Grookey: 24-30 (114.2 - 142.8%) -- guaranteed OHKO
Foongus is the best Poison-type counter for Grookey, Salandit is a nice check although it seems to take very less damage.
236 Atk Life Orb Grookey Wood Hammer vs. 0 HP / 0 Def Eviolite Salandit in Grassy Terrain: 5-6 (23.8 - 28.5%) -- 3.7% chance to 4HKO after Grassy Terrain recovery
Note that Salandit has a weakness to Stealth Rocks, which can leave you pretty vulnerable.
Koffing also works, although getting 3HKOed by Wood Hammer despite Eviolite and not getting any recovery can put into a disadvantage.
Woobat carries Grassy Seed for to counter Grookey. Simple gives Woobat a +2 Defense boost, which despite its mere Def of 43 allows it to dodge a 3HKO nearly.
236 Atk Life Orb Grookey Wood Hammer vs. +2 76 HP / 252+ Def Grassy Seed Woobat in Grassy Terrain: 5-8 (20.8 - 33.3%) -- 0% chance to 3HKO
Trubbish tends to get 2HKOed without Eviolite by Life Orb Wood Hammer, which is pretty threatening. Note that it need to carry Recycle + Berry Juice for recovery, leaving no space for Eviolite.
236 Atk Life Orb Grookey Wood Hammer vs. 116 HP / 100 Def Trubbish in Grassy Terrain: 12-16 (52.1 - 69.5%) -- guaranteed 2HKO after Grassy Terrain recovery
But since the damage output causes it to use Berry Juice, it can swiftly get the OHKO on it, or Recycle the item back.
Oddish, Stunky and Skorupi are some other options. though note that any poison type doesn't lose to Grookey can be used in the team. Rowlet, Ponyta and Bulbasaur are good options as well.
Tl;dr this are the most common counters. Note that for checking Grookey, any Pokemon that can outsped it forcing to use Grassy Glide, can survive one hit then land the OHKO can be used, but they aren't counters all the time since they always need a free-turn to switch and don't get recovery as always.
That's all, hope this helps.