Depends on the type of Geo you are after. The Brasiliensis types are ok over gravel, they are not the sifters that the Surinamensis types are, for them, I would say that sand is prefered.
It all depends on what you feed them. My Suri being matured and accustomed to eating the pellets I feed the rest of my fish, gets them easily in gravel. If you crush up pellets, use smaller ones, or flakes, and they need to sift through it to get to them, it is going to much harder..