Gs3 reads the 2.5 .gig files very well -- faster, in fact, than it reads version 3 gig files, and significantly faster than 2.5 reads the same gig files. No problems. The editor can even save 3.0 files INTO 2.5 format, if you ask it to. With 3.04, the patch changes work well too -- that caused a big disruption with existing projects, with the early revision of gs3.
The only problem remaining in gs3 (for me) is stuck notes when midi information gets too thick.