As lempkee said you should take a look at your install disk that came with the GVP harddrive. There is a program on it called Faaastprep, which you just double click and it will load. Choose "Binddrivers" from the pull-down menu, and you should see the activity led light up. Now just select automatic or manual setup, and you should be able to set the partition sizes, format partitions, copy files etc. Just remember that you should set the switch on the harddrive to autoboot, or else you won't be able to recognize the disk.
Have fun
@lempkee
There is actually a HDToolbox for OS1.3. It can be found on the install disk for the Commodore A590 harddrive, but pretty ugly with only 4 colours.
-- EDIT --
Just read your post again. The install disk can be found here
http://www.gvp-m.com/a500hd8.dms . You would of course need to extract the disk before you can use it.
