If you have done all the things that David C has suggested and it still won't open, do the following.
Open the "ski hatch" in the rear seat and shine a light onto the latch of the boot and try to open it with a long stick. Once open, you can inspect and fix the problem.
If that does not work, simply remove the rear seats (four bolts, if I remember correctly, perhaps six) and you'll have easy access to manually open it from the inside and inspect and fix whatever is the problem.
Also, have a look at the South African lounge on this forum. When did you get the GTA and whereabouts are you located?
South Africa - Alfa Romeo Forum