The idle speed on your GS500 should be 1200 rpms when the engine is warm. Suzuki recommends an idle of 1100-1300, with 1200 as the ideal speed. To adjust the idle speed, first warm up the engine with a 15 minute ride. Keep the engine running and twist the idle adjust screw until it hovers within the 1100-1300 speed. To find the idle adjust screw look on the bottom the carburators above the oil fill hole.
Gloves are recommended since the cooling fins on your cylinders will be pretty hot. Turn the screw to the right (clockwise) to increase the idle speed, and to the left (counterclockwise) to decrease it.