Technically, it is possible to transfer down Volt Tackle from a Gigantimax Pikachu. However, the Pikachu that is being used for breeding must hold a light ball. Without it, Volt Tackle remains an inaccessible egg move.
If you're unsure where to get the light ball, it can only be obtained on Pikachus on Route 4. It is a 5% chance, so I'd recommend using Compound Eyes or Frisk, then Thief to secure the item once it is found.
I hope I helped!