The problem with Gorilla Tactics is the fact you'll probably have to be switching constantly, especially against foes without Monotype teams. You can teach it U-Turn (TM found in Glimwood Tangle) to help with that though.
Galarian Darmanitan already has 140 base Atk, and in-game, that's really enough. Not to mention that coverage is usually more important than power (not that it needs any more) in in-game playthroughs.
Zen Mode Galarian Darmanitan's extra Fire-type STAB will be extremely useful, because most Ice-types can't use Fire moves and you're doing an Ice Monotype. This extra Fire coverage/STAB can help against foes like Leon's Aegislash and Mr. Rime/Rillaboom (If you choose Scorbunny or Sobble), Opal's/Bede's Mawile, Melony, and Hop's Corvikight.
If you want to take the time to breed it, Zen Mode is probably better for the extra moveslots in battle and potential Fire STAB.
Besides that, it can learn Belly Drum to immediately activate Zen Mode.
Darmanitan-Galar @ Charcoal / Never-Melt Ice
Ability: Zen Mode
- Belly Drum (Learned at Lv. 38)
- Icicle Crash (Learned upon evolution)
- Fire Punch (TM bought in Pokecenter outside Wyndon)
- Zen Headbutt / Iron Head / Earthquake / Anything Else (TRs)