Hi Frank
You will need a Shoestring Astronomy, GPUSB:
http://www.store.shoestringastronomy...roducts_gp.htm, scroll down the page.
Attaches to a USB Port on your Laptop, the guide cable then goes to the Autoguider Port on the Gemini Controller. The cable must be ST-4 compatable, if in any doubt, Shoestring have the correct cables.
PHD and your guide camera will then autoguide your mount.
(Make sure you are in Photo or All Speeds Mode when using an autoguider)
The RS232 Port has a different plug type, you use this Port to connect your computer control to the Gemini, you then can perform GOTO's using various sky programs, it usually has a DB9 plug on the computer end.
Cheers