It's best to replace the stock springs, and change the oil while you're in there. New springs make a night and day difference on the GS500, and are an important safety improvement since having the wrong springs compromises braking and cornering traction. But if you're stuck with the stock springs, adding some preload to get your sag measurements correct is definitely a good idea. Replacing the oil, especially with something thicker can help.
But seriously, don't cheap out here - even if you bought your bike for $500. New springs and a shock won't cost you more than $200 if you do the work yourself, and will completely change the nature of your GS.