The engine in the gs is completely different. The gst engine is a mitsubishi 4g63 and the gs is a chrysler made 420a the only similarities are they are both 2litres. to do the conversion the only thing that would be good off the gst is the turbo itself.
you would need to get custom piping made an intercooler, oil lines, an upgraded fuel pump, a custom manifold, and a boost gauge and a lot more. But the other option is to go to star,hauhn or ner and get a turbo kit which will come with everything you need to convert your car to a turbo. they start at like $1700 for a stage one. also go to
www.2gnt.com and look in the forums there are a lot of guys that are running turbos in their gs and rs's. They should be able to answer your questions better.