Very easy. The instructions are fairly basic, mainly because there's not much to say.
I'm going to assume that the layout of your GS is similar to the hexhead and the connectors to the O2 sensors on your exhaust live underneath the cylinder heads.
On the hexheads, there's a plastic cover over the connectors fastened with two screws. Just remove it and the connectors are easily spotted. Unplug the connector and plug the AF XiED unit in line with it. It's impossible to get wrong!
Have plenty of nylon cable ties to hand.
I used a few to keep things tidy at the connectors. There are some pics on the thread I had started about fitting them.
http://www.ukgser.com/forums/showthread.php/418381-Standard-vs-PCV-vs-AF-XIED
Once you have things tidy, run the cable back up to a suitable place to mount the units themselves. They're not very big and I've put a pound coin in the pics to give an idea of size.
Use more cable ties to keep the cable routing neat to prevent it snagging the throttle cables.
The units are then plugged into the cable once you find somewhere to mount them. I just used some double sided tape to mount them.
The only other connection is a good earth for each unit. I had read that the best earth is directly to the battery which is where I terminated mine. The units are powered by the O2 sensors feeds.
Most people run them at 7 or 8. Zero being stock. I run mine at 8. The instructions detail how to read the settings.
Remember, it'll take at least a couple of tanks of fuel for the ECU to adapt. Beemer Boneyard reckons a thousand miles.
I always thought the changes they made made my TC feel something approaching the LC that I had taken out on a test ride, so would be really interested to hear the effects on your GS.
Please let us know what you find when you try them.