The GS500 stock battery is a Yuasa brand #YB10L-B2. The specifications for the stock battery are 12 V and 11 A-h (estimated cold cranking amps are 62 A). The stock battery is not very common because it has its vent hose on the right hand side.
The dimensions of the stock battery are:
Official specs: http://www.yuasabatteries.com/battery.php?bID=B39&vID=3994 Official charging amps = 1.1
A similar battery is available with the vent hose on the left hand side (Yuasa YB10L-A2). This alternative is more common than the stock battery: less expensive and easier to find at common suppliers (WalMart, auto parts stores, etc). Because the battery vent is on the opposite side, the vent hose will have to be re-routed along the left side of the motorcycle. With the battery vent now on the left side, overfilling of the battery may result in spilling of acid onto the motorcycle swingarm when the bike is on the sidestand (i.e. leaning to the left).
If a sealed "maintenance free" battery is something you're interested in, the BikeMaster TruGel, part number MG10L-A2, has been confirmed to work great in the GS500. Type it into Google, find the best price and go for it!