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It should fit, same bolt pattern. I think it will just be inset more since I think the stock IS wheels are 17x7 and the GS are 17x7.5. I'm just going from memory though so I might be off by .5.
I think Rockero28 had IS wheels on his GS for awhile.
i can confirm it will fit. stock gs is 8 +50, stock is3 is 7 +50. it will fit over stock calipers no problem, the wheels will be inset more but that's fine. if you just want to get the car back to stock and sell, it's a solution
basically it's like the stock gs17 but quite a bit narrower and also sticks in more. keep in mind in the last thread, the is3 wheels are on spacers already so it sticks out more