Yeah, GS wheels will fit. A good number of Lexus cars have the same PCD and centre bore size (5x114.3, 60.1mm), so those from other models will also fit an IS.
Including the OEM wheels my car came with, I've had 4 different sets on mine and currently, I alternate between these two.
18" MY2010 "Hoshi" F-Sport wheels (summer).
18" 2013+ 3rd Gen IS F-Sport wheels (winter).
Hard to say which ones I prefer as I always seem to think they look better than the other set each time I swap between them, but if I had to narrow it down, I'd pick the Hoshis. Especially in the OG titanium-style finish.
I also had a set of 19" IS-F wheels for a while, but was never really endeared by them.
My previous car had the optional "Baransu" (aka G-Spyder) wheels. Of which I was definitely a fan.