Buccal I think you need some professional therapy mate - wtf does that even mean?
But Jules he is right - you can keep geo's without substrate - but why would you want to?
You have to read slowly and think hard, be patient for the wisdom to enlighten you
Usual reasons for bare bottom is for sanitary reasons.
Keeping Geos with other species, may call for no substrate because some species do a lot of fouling especially in many TB situations.
Another is,,,, when keeping a fair whack of tanks,,, bare bottoms are undoubtably time/life savers.
With no substrate less water flow is needed to for a gentle circulation to still be effective in keeping debri of bottom and having no dead spot collection points, (a smooth unobstructed flow over the glass surface).
These are usually reasons for bare bottom apart from slimline presentation of the fish (to be all about showing the fish).
Bare bottom doesn't seem to effect geo breeding, but I take fresh wriggles and nurture.
There could be complications the natural way as they like having a series of pits they like to transfer fry from one to the next.
Sandsifters (tangs) can breed without substrate, but I have found two or males with no substrate and no pit to build creates to much confusion and doesn't go so well.
With your earth eaters,,,, just don't forget to use full spectrum lighting that allows all colors to shine.
There are a lot of intricate colors in Geos that are missed with incorrect lighting.
Im a fan of the T8 power glo. In the pink/purple box.