The driver you need is for a built in chip that acts as a USB to serial convertor in the GT-31. So once it's installed you will just see a COM port that is used to transfer data to and from your GPS. Just plug in the USB cable that came with your GPS, that's all you need.
If you plug in the USB cable into the GT-31 and then the laptop, it should usually install a driver automatically. However, depending on the Windows version (and I have no experience with Windows 8 yet) it might not work out of the box. I do know though that the driver you're looking for is for the PL2303 from Prolific. Probably downloadable here: http://www.prolific.com.tw/US/ShowProduct.aspx?p_id=225&pcid=41
Once you have that installed, reboot your PC, plug in the GPS again and try again following Tom Chalko's guide. Now you should actually be able to select a COM port.
Best of luck.