JG,

On gravity filters.  You don't say which MSR filter you have, but if it is the MSR Hyperflow, you already have a gravity filter and a pump.  They sell a gravity kit for the Hyperflow, but you don't need that, just a way to attach it to a supply bag and run the outlet to a reservoir of your choosing.  I use a  modified 'camp shower' bag for the supply bag.  I put in a length of tubing, plus a camelbak on/off quick connect adapter to make hanging and setup easier.  It will self start, but a few pumps get it going pretty good.  It's easy to do some searching on it for info on the web and check out MSR's site for info too.  Pump or gravity - it's nice to have the option.

pd