Oh, and, yes, a lot of people here prefilter their water through a bandanna if it's silty. Another lightweight option is just bringing some paper coffee filters along.
I'm minimally familiar with the platypus gravity filter system, but I've heard others here deride it as too heavy. Or maybe I'm thinking of some Camelback system...? I'm not sure. But I KNOW you can make a lightweight system yourself. Someone here posted about one...
EDIT-- can't find it. But here is the reader reviews for the Sawyer. Tony Wong is a bit of a Sawyer pimp like me and he goes through making a gravity system (though Sawyer markets their own, now, but it's heavy). Tony's rig has one faux pas- the filter should be closer to the CLEAN water bag, not the dirty one- so that the higher water pressure makes for a faster flow.
Though Tony's pictures look a bit, er, primitive bear in mind that a lot of the functionality he went to great pains to produce is an out-of-the-box funtion, now. See my review below his. I've also heard that the newer Sawyer filters have quick-release fittings, too, so it is even easier to share the same filter and use it many different ways if you want to.
Here's the Sawyer rig; not sure what it weighs:
They have 4L and 8L versions, too. (And you can, of course, just carry 2L of water "dirty" in the dirty bag, too, for a total of 4L.)
Also, Tony reviews the Sawyer rather harshly, but in later posts he has come to love it a little more, and I think uses it almost exclusively. Plus, it has a lifetime warranty! You never have to change a filter element again. Ever. For life.