You cant put a 1999 WRX
ECu into a 1997 GX. It wont plug in.
If you put a 1998 WRX
ECU into a 1997 GX (Which im pretty sure will plug in). It probably wont work properly if at all. Assuming it even manages to start your engine, it will throw a CEL for the missing turbo components and sensors. Which would send the
ECU into limp mode and cause a massive decrease in performance over the stock GX
ECU.
If you want to make your GX go faster. buy a 1998 WRX front cut and get a pro to swap the engine, g/box and wiring over.
In the end, it would be much much better to simply save your dollars and sell the GX and buy a WRX.