I wasn't thinking about the ObjC frontend; the ObjC frontend, AIUI, was contributed by NeXT before NeXT became a part of Apple.
If you go grab the GCC tarball Apple provides, it has many, many changes from the GCC 4.2 sourcebase. Lots of those changes have not been submitted back upstream. The code is available, granted, but it's not available in a way that's helpful to anybody but Apple.
So? They complied with the GPL. Why is it that people also expect companies to push code changes back upstream, maintain said parts of code upstream and properly document said patches?
If upstream wants patches so badly they can download the tar-ball provided by Apple, diff the two source trees and document, and submit those patches to the upstream. It is not the companies responsibility to do so. The GPL only states that it MUST make the source code and its changes available.
If you're talking about the NeXT ObjC frontend, that wasn't a matter of course. It actually took legal threats to get them to meet that obligation.