Work-Around
Please note: I did this experiment before I owned a gravity fed airbrush. Since then, I have been fortunate enough to have tried a variety of them. My experience agrees with the results I found here. A gravity fed airbrush has little or no advantage for fine lines over an equivalent siphon fed brush with a 1/4oz. cup. The key word here is equivalent. It isn't fair to compare a .5mm siphon brush with .35mm gravity fed.
I decided to see how much difference a gravity feed actually makes. All that's required to make a siphon feed brush act like a gravity fed brush is to create a slight positive pressure in the paint chamber, and it turns out that this is very easy to do. All that's needed are some supplies from the pet store, a plastic bottle, and a cork that's the right size.
This is an inexpensive aquarium air pump, some aquarium tubing and fittings and a cork with a hole in it. The cork is hardest part to obtain as it has to fit the paint cup. This one is from a bottle of Weston Chardonnay. Wine corks are not all the same size. The bleeder valve is necessary because we're not talking a lot of pressure here. Most of the pump's output will be bled off. The bottle acts as an accumulator to smooth out the pump's air flow.
Here's how I calibrated the bleeder valve. See the picture on the right. I put some water in the paint cup. I opened the bleeder valve all the way. I plugged in the pump. I adjusted the valve until water rose in the clear tubing an inch or two above the rim of the cup. You don't want too much pressure or it might force paint pass the needle bearing and into the rear of the brush. Here is the brush ready for use.
In use, I found that the brush would spray nicely with pressures as low as 10psi. However, I wasn't able to obtain lines any finer than what I would normally be able to produce by merely increasing the air pressure to 25psi. This may have something to do with my limited skill, but I'm beginning to wonder how significant a difference a gravity feed actually makes. Notice that when spraying in a downward direction, the vertical offset of the airbrush nozzle and the bottom of the paint cup is less than one inch. So even without the added pressure, not much vacuum is needed to raise the paint to the level of the head. I suspect that the ability to produce fine lines has more to do with needle and tip design and dimensions than with the type of paint feed.