So what is the advantages/disadvantages of the different feed types?
mp wrote:

The gravity fed hvlp guns (with the cup on top, as seen in car body
shops) require much less air and probably can run on your compressor.

Actually, HVLP guns consume a lot of air (High Volume Low Velocity) and
the type of guns used in body shops typically require an industrial-sized
compressor. Also, the type of feed doesn't affect the air volume needed to
atomize the finish. Many manufacturers offer the same model in gravity
feed, siphon feed, and pressure feed, and the air consumption is the same
for all configurations.