Essentially to run something like Gnome on a different platform than GNU, you're porting it. Most people consider something as low-level as the x86 standard a "platform" (Java itself is considered "cross-platform"). I don't see as clear cut of a difference between GNU, XFree86, etc. and something like x86, Java, KDE, etc. (btw, you can run a KDE prog just fine in Gnome and vice-versa, as long as some of the underlying components are there).
The point is, that has nothing to do with it. Gnome is a GNU project, and Gaim stands for "GNU AOL Instant Messenger". Just because something starts with G doesn't mean it's specific to Gnome (i.e what do you think "glibc" is?)