Tl;dr

Grimmsnarl probably isn’t worth the grind. Bring a steel attacker like Zacian or Metagross and some combination of Zamazenta, Zacian, Blissey to tank.

Gigantamax Grimmsnarl will be available globally on November 15^th and 16^th. Grimmsnarl has a reasonable attack stat and is our first Gmax with a Dark type Max Move. However, in almost all cases Gmax Gengar will be a better attacker than Grimmsnarl. If you have a Gmax Gengar then Grimmsnarl might as well be just a dex entry. The only cases where Grimmsnarl gets ahead of Gengar is against a boss that is either Ghost & Normal or Psychic & Normal.

So it’s not a groundbreaking attacker, what about as a tank? It has many 0.5s fast moves to fill the Max Meter quickly and three double resistances. The issue is that Grimmsnarl isn’t bulky. Against moves that Grimmsnarl double resists Blissey would survive longer.

Attackers

Zacian is almost certainly your best option, even if it isn’t maxed out Zacian is so far ahead of the crowd its hard to have a better option in your collection. Even with a weather boost it is hard for the best Fairy or Poison types to catch up to Zacian.

Tanks

Grimmsnarl was given a lofty 1.2 CPM with the usual 0.9 attack multiplier, combined with the attack stat Grimmsnarl has it will be hitting pretty hard.

Grimmsnarl’s has a very average collection of charged moves, nothing super weak like Acid Spray or hard hitting like Blizzard. If your Zamazenta has Max Guard at level 2 or level 3 it’s the clear winner. Power-Up Punch does super effective damage to both Zamazenta and Blissey but it’s the weakest power move and Grimmsnarl doesn’t have STAB on it either so it’s tolerable.

If you’re in a small group and need to reroll for the most beatable moveset try to get both Foul Play and Dark Pulse, then running Zamazenta and Urshifu Single Strike as your tanks makes the battle very predictable.