Gigantamax formes and moves should not be included in the data files (they are not in the game data files, they are not formes/moves by that definition). PS has begun doing some work here, the most obvious being marking the 'formes'/'moves' as Nonstandard and adding Gigantamax: Yes to the importable. The following is all required:

• support Gigantamax: Yes in imports (moveset_import.js)
• remove nonstandard Gigantamax from being special cased for inclusion
  • similarly, clean up otherFormes hardcodes (in particular, Toxitricity/Urshifu) to remove the Gmax 'forme'
• Gigantamax should be a checkbox in the UI instead of a 'forme'.
• Set import logic should switch to sets with gigantmax: true instead of the legacy -Gmax name approach

The main question is around moves - how should the Gmax version of a move be handled? This should probably block on PS implementing the Gmax data as a field on the base move.