Gravity does play a role in the process. Dennis had mentioned it in a prior interview (or maybe the conference call a couple months back).
Anyway, I actually do think there's a combination of gravity/pumping. If I remember correctly, the base is built with a slight grade toward where the algae exits the system for processing (the lower tank that we cannot see at the SJC demo unit).
The pump then takes the water back to the upper level to be used again in the system. The pump also helps the water flow in the tubes, but there is a combined effort of pumping/gravity due to the thin slope. Don't know how it splits percentage-wise, but to some degree both play a role.