The retail Game of the Year edition came with the soundtrack (included with GOG's version), a printout of an in-game tabloid called Midnight Sun (included as a .PDF with GOG's version), and the manual (again, included as a .PDF with GOG's version.) Not sure if there was anything else.

GOG's version further includes a reference card (PDF), some HD wallpapers, and avatars sized for the GOG forums. It's also pre-patched to v1112fm.

If your retail version still works, there's really no reason you need to buy it from GOG unless you want the extras or want to support their business model.