I did three brews with my plate chiller (ChillOut MKIII) using gravity, before getting a pump. I had to invert the chiller so the name plate was facing down, in order to get it to flow at all. With my rig, the drop from kettle to wort in was only a foot or so. Others have commented they use gravity only without issue, but with a good 2 foot drop (to wort in). Lots of variables here, which could affect flow rate. Hard to advise.
In my case, flow rate using gravity was less than spectacular. With the addition of a pump, I reckon it's the best chiller I've owned. I wish I'd skipped immersion, counter-flow, etc. and just went straight to using a plate chiller.
Hope helps,
reVox