Edit: After typing out my response I realized you want to use GC parts. I know for sure that GD shocks and springs bolt up but unfortunately I'm not familiar with the GC stuff.
GD parts will work, but with Impreza lowering springs it's going to be pretty low. You'll end up with moderate bump steer, manageable but not good for spirited driving. To do it properly you'll want to get a Whiteline roll center adjustment kit. This will help correct the messed up geometry caused by dropping it so much. If you have a manual, you can also remove the front subframe spacers to fix the geometry, but for auto cars the spacers have to stay in place. You'll also want to add spacers to the rear shocks, otherwise the car will sag in the rear. This is especially important if you're using Impreza sedan springs. Wagon rear springs are slightly stiffer but may also require spacers,