Is it possible to put an engine from a 3000gt into a GSX?

Anything is possible, however it would seem pointless to do so. The 3000GT Engine is much larger & weighs much more. The modifications needed to do so would not be worth the effort. The original 4G63 engine in the eclipse will take you a long way. With proper modifications these engines are capable of 1400hp. So to answer your question, yes it is "possible" would anyone with experience recommend it "NO"