Just to add to Keith's post, the GT is a very small car, often smaller than you think because the proportions tend to hide it, and the old Opel CIH engine is actually a fairly compact engine and because it's a short stroke engine it's not as tall as many more modern engines, so while it may appear to be some space in the engine bay for other engines you'll find that many engines will require some modifications of the engine bay to actually fit
The more obvious alternative is the 2.4 version of the engine already in there, an extra 500cc of volume and an honest 125 hp in stock form, that's about 50 hp more than the stock low comp 1.9 you find in most GTs. Combine that with a 5 speed conversion and you'll have a pretty quick car that will also be very relaxed at highway speeds
The downside to the old Opel engine is that if you'll want more than 150 hp or so out of them they tend to be fairly expensive to modify, in which case you're probably better off starting with some Japanese based turbo engine
Personally I would have gone with a stock or near stock 2.4 and a 5 speed, the GT weighs next to nothing and it will be quicker than you might think