And Mac with X11 emulator/vm/what ever XQuartz is. And Windows if you use the 2 year old version that has a ton of issues (UI bugs, glitches, old GTK bugs, etc).

Running on multiple Linux DE's/OS's is not cross platform as far as I am concerned, even though technically it is lol.

That's not what people mean when they say "cross platform".

"Cross platform" means supporting all of the widely-used platforms at a given point in time, and supporting them well. Today, that includes at least Windows, OS X, and Linux. Some would even extend that to include the BSDs, Solaris, AIX and HP-UX.

Like others have pointed out, GTK+'s support for OS X and Windows has been very, very lacking. It's nowhere near as seamless as that offered by Qt, Swing, or SWT.