I have used a gravity filter for a number of years, first the ULA Amigo Pro (no longer made) and then a homemade version using the same filter (Katadyn Hiker Pro). No pumping, just hang it from a tree and keep an occasional eye on it so your water bottles don't overflow.

I recently switched to the Sawyer Squeeze, but (thanks to a foot injury that kept me off the trail last summer) haven't been able to test it in the field. However, in at-home tests in the bathtub, it works great as a gravity filter. I will ditch Sawyer's fragile squeeze bags (pretty useless, per reports) in favor of a 2 L Evernew water bag (the newer Platypus containers don't fit the filter threads, although the older Platys with straight sides do).

Weight of gravity filter setup (Evernew 2L bag with cord for hanging upside down, filter, tubing to connect to water bottle), with wet filter: 5 ounces
Filter size: 0.1 micron

Since this filter removes all bacteria and protozoa, there is no need to boil or chemically treat unless there is human habitation upstream. In that case, viruses can be a problem but are quickly zapped by chlorine dioxide pills, far lighter than carrying extra fuel. (The 4-hour treatment time on the ClO2 pill labels is for protozoan cysts in extremely cold and turbid water.) However, if you always boil your water, why bother with the extra weight of a filter?