Rearsets are footpegs that are moved more to the rear, than the stock ones.
At one point, there existed ones for gs gs500 made by "woodcraft", as referenced by http://gstwins.com/gsboard/index.php?topic=42757.0 However, the company does not seem to list the gs500 as current production item. Not to mention that it only moves the position a mere 1x1 inch
"The Gimbel rear sets move your feet back about 4" and up about 2".
There are also Raask ones available from some places.
Beware of the "tarozzi type", eg http://www.discovolantemoto.co.uk/tarozzi/model-specific-rearsets/suzuki-rearsets/suzuki-gs500-gs550/prod_758.html WARNING: these are anti-recommended. they do not fit all except 1980 models.