"I still haven't tested it, but I found the other half (and possibly a newer version of the first half) here:
ftp://ftp.gravis.com/Public/Pcstick/GPPRO_D1.ZIP
and here:
ftp://ftp.gravis.com/Public/Pcstick/GPPRO_D2.ZIP
You can extract both to one directory, but extract D2 first, or choose not to overwrite the like-named files.
BTW, the "Poor" rating is because the installer requires a reboot. Yeah, for a freaking gamepad!
Also, if the switch on the bottom is set to "i" (and it's plugged into a standard game port) the GamePad Pro works with the built-in Windowz GamePad driver, but the extra buttons do weird things..."