If you want a correct denomination for Gaia planets it would be something like "paradisiac" or "super-habitable" world. But "Gaia" has at least two pratical uses:
- it's short
- it's vague enough to fit in any ethics/logic. There's a bit of mystery (it evokes the "Gaia hypothesis", and in fact several mods that adds events will feature at least one sentient Gaia planet), but it doesn't sound as scientific as "super-habitable" either. Contrarily to "paradise", "eden" and other denominations, it doesn't imply necessarily that the planet has to be a protected garden (or a resort world of some sort) - it only suggests it has the potential to be one, because you can totally make a Gaia planet a death world.
And when it comes to spiritualists - interpreting Gaia wolrds as sacred worlds is just their way to recognize the particularity of Gaia worlds, but like I said, Gaia worlds are a bit mysterious for everyone. Materialists will probably say something like "there was statistically a chance against 5 trillions for things to be in this way, it's like the stars aligned to allow this planet to exist", while Spiritualists will think something like "it's a gift from the gods/spirits/veil/Money/whatever mystic stuff" (but not a "proof", because Stellaris' spiritualists don't seek materialist recognition).