I looked around for any good pokedex entries that could potentially be used for AP scaling and found
this entry for Gigantamax Corviknight:
Imbued with Gigantamax energy, its wings can whip up winds more forceful than any a hurricane could muster. The gusts blow everything away.
According to this
page Hurricanes are considered 8-A, High 7-C with cloud formation.
This places any Gigantamax at High 7-C, at least.
However, Lugia is considered to be Low 6-B due to its 40-day storm feat (
calculation here). If Corviknight's winds are more 'forceful' than 'any' Hurricane, this should also include Lugia's Hurricanes that are Low 6-B.
Thus, I believe if we were to take this dex entry seriously, every Gigantamax should scale to being at least Low 6-B.