The Gaia Market building can only be seen maps like Marketplace, and is non-existent. Such a building being hidden doesn’t really make much sense since it could be used as a good asset for potential scenarios. I suppose you could technically get the building if you copy and paste part of the map if you create a Marketplace map using the map generator found in scenario editor. But even then that’s not very convenient.
Edit: I done so here. But even then, this is very inconvenient.
