Hi mate,
The stud pattern for the GT is 5x98, therefore any wheels from the 147, 156 should fit, just be cautious of the offset, I believe they have an ET of 35 from the factory on a 7 inch wide wheel.
Newer wheels from more modern Alfas such as the 159, 166, Giulietta, Mito etc will not fit as they have diffent stud patterns.
After market wheels will fit, I would probably estimate you could get away with an 18' by 8' wide wheel with an offset 25, that way the wheel will sit flusher with the guards and still be away from the inner wheel liners.
Another alternative is running 'wobble bolts'. I haven't had experience with them, but seem to be recommended quite a few times, which can allow a 5x100 wheel to be fitted. This means there is a wider variety of wheels as Subaru uses this stud pattern.
My opinion: source a nice set of Alfa factory 18' wheels and be done for, that way you can be sure they will fit without any rubbing issues and can maintain some of the cars originality.
Hope this helps,
Andrew.
