Go here: http://adtpro.sourceforge.net/ and download a copy of ADTPro and then either make yourself a cable to run between your modern computer and the GS or order one from here: http://retrofloppy.com/products.html and you'll be able to transfer Apple II disk images that you can find online, such as here: ftp://ftp.apple.asimov.net/pub/apple_II/ or here: http://www.whatisthe2gs.apple2.org.za/ ore here: http://macgui.com/downloads/?cat_id=53, back to real disks to run on the GS. Another option would be something like this: http://dreher.net/?s=projects/CFforAppleII&c=projects/CFforAppleII/main.php that you can install in the GS and then use either CF cards or Flash Drives to run the old Apple II game disk images from.

Hope this helps.