Well, if it is a genuine WRX turbo, it can be done for $1200, plus a new tranny, clutch, axles, andd hubs.
The GL motor is stronger than some people think, although I don't recommend running 15 psi or anything like that.
True is it weaker than the GT, but it does have the same crank, and decent rods. The pistons are the same type of casting as in a Kia sportage, and they can take to boost pretty well.
If you really are looking at the true WRX turbo, 4-6 psi with the higher compression will make you feel almost NO turbo lag, and pretty good horsepower numbers. BUT, everything behind the engine won't hold up to it very good, so you will need to upgrade the drivetrain.
Also, the coputer will get confused, the injectors are too small, and you don't have intercooler piping, or an intercooler.
The GT's also have a oil cooler that you might want to look into, I don't know for sure if the GL's have them or not, but I've heard that they don't.
If you do just hook it up it will go BOOM!! So you will need to do something about the computer.
I can see this thing done for about 3000-3500 bucks, though, and it would be a real surprise to anyone pulling along side you. In fact the turbo will come on so quick, that they would probably assume you still N/A, too.