Generally, one can grow any brake pump (master cylinder) to the correct bore / stroke ratio and the corresponding connectors. However, it is much easier if you used the same series a fitting part. In my opinion, the GS 500 F would also have all of the Master Cylinder GS500E (GM51A / B) match.
You can even compare them out:
http://www.alpha-sports.com/suzuki_parts.htm
same part numbers = same parts.
A used brake master cylinder you can get on the Internet, for example ebay. or you try used parts dealers.
There is also the possibility here in the forum in the marketplace looking for the "Offer Share". Maybe you can find here the part.
http://forum.gs-500.de/forumdisplay.php?f=107
for example
http://forum.gs-500.de/showthread.php?t=31278 (Tokico Bremsanlage vorn, komplett)
front brake system completely