FuelFilter
GS500 does not come with a fuel filter from the factory. It's easy to splice a fuel filter into the fuel line going from the petcock to the carbs. Lots of topics on GSTWIN.COM board about that, try this one.
Make sure to use appropriate fuel filters 'designed for motorcycles', like this one!
Many people prefer using such transparent (clear) filters. You can easily see if there's gas in the system. If there is water in the system it will tend to sit at the lowest point, i.e. in the filter. With this fuel filter you can easily spot water in the fuel supply.
A word of caution using car fuel filters on carburator motorcycles. A lot of people (including myself) have gotten in trouble by putting the wrong kind of filters on GS500 (i.e. filters with paper filter elements). Most automotive filters assume a LOT more pressure, coming from a fuel pump. Fuel-injected motorcycles also have fuel pumps. We just have gravity feed with a drop of, what, less than a foot? The pressure drops as the fuel level drops, so sometimes this is "cured" by filling the tank, not because it was empty but because the weight of the gas in the tank increases the pressure in the hoses, and the owner ends up thinking that they just have an awfully high "reserve"... for a while, until the filter gets a little more clogged and the situation worsens.
I found a great place for the filter -- under the petcock (see picture). The filter is injected into the fuel line from the petcock to the carbs. The hose is secured to the frame with velcro strip and that keeps the filter tight. It's easy to see if there's anything in the filter.
When installing remember about direction of gas flow (marked with the arrow on the filter).
UPDATE. Two seasons later.
I swapped the old filter with a new one of the same kind. The old one held up well and performed fine. It got deformed from all the heat and the pressure the curved hoses placed on it (see the comparison picture), though again performed great. Now I re-routed the hoses and hopefully the new filter will stay straight.
I opened up the carbs and there was virtually no junk in them after two seasons. But I treated my tank with POR-15 long ago. Remember, if you have junk and rust in your tank no fuel filter will save your carbs from being clogged up. Ask me how I know.
Amazon Stens Filter
Installed a fuel filter from Amazon, Stens 120-436 (~$3.50, free shipping for Prime members) designed for garden power equipment. I was worried that it wouldn't flow well enough for our gravity fed system, but was assuming that a lot of large garden equipment didn't run a fuel pump anyway. As I was putting it in, I found that the PO used standard PVC tubing for the fuel lines, they were rock hard and even had "FDA approved" printing on the side. I replaced it with thick 1/4" ID black fuel tubing from Home Depot made for garden equipment. Mounted the fuel filter roughly under the tank on the left by the petcock for easy viewing.
I didn't have properly sized hose clamps, so I'm running without them for now. The 1/4" ID tubing is a little smaller than the stock 7mm tubing, so it fits fairly snugly. The filter was designed to attach to both 1/4" and 5/16" lines, so I pushed real hard and made the 1/4" line squeeze over the 5/16" barb for extra holding. Took it up to full throttle for a while and no fueling issues so far after a tank. You can see some of the old fuel lines in the picture below. The tank is still off in the picture, but when it was all done, the filter was positioned a but further back.
Edit: I've since replaced the thick black reinforced lines with Tygon Lines: http://wiki.gstwins.com/index.php?n=Maintenance.ReplaceFuelLines
