Not sure whether the Gstar has an RS485 or an RS232 port. The camera itself could come with either, with the manufacturer supplying RS232 as an option. Since Binary Systems do an OEM with the GStar manufacturers, the RS232 option would probably be in there.
The cable from Steve Massey costs AUD65 which kind of suggests there is no 485 circuitry involved. An IC, the 75176 would be needed as a minimum - see this page for a circuit:-
http://www.airborn.com.au/layout/232to485.html
such converters are apx AUD50 to do yourself and over AUD100 to buy premade.
If you are not very conversant with electronics, I seriously suggest that you get the cable from Steve Massey. You will have assurance that it works. I presume from your post that you have the cable.
An RS232 to USB converter will be available from many sources, Keyspan may well be the best (though pricey) but I have used generic PL2303 devices with success (using the Prolific chipset and drivers).
Regards,
Tony Barry
(Disclaimer - I am not affiliated with Steve Massey or MyAstroShop or Binary Systems, although I am looking seriously at their camera).
[edited for spelling]