There have been three generations of the GS.
The first generation GS was sold from 1991 until 1997. It was sold with three different engines: an I6, a twin turbo I6, and a V8. The first model was the GS 300 and it replaced the LS 400 in the market as that more luxurious car moved into a higher price territory.
The second generation was sold from 1998 to 2005. It had the same engine options as the previous model. The American market now got a GS 400 with a V8 engine making 300 horsepower, giving this midsize car quite a good amount of pep. The GS 300 was still around and got the inline 6 cylinder engine.
The GS 400 got an upgrade in 2001 and the engine was made more powerful. The model name was changed to a GS 430.
The third generation models have been sold since 2006. The GS 300 has been sold with a 3.0 engine, the all new GS 350 is sold with a 3.5 liter V6 engine, while the GS 430 is sold with a V8 engine.
The latest generation GS 350 sells for about $44,000, while the GS 400 sells for about $54,000.
These cars generally do not show up on used car lots, but you can find them used if you know where to look. Try checking on eBay.