That's just patently untrue: either GTk or Qt would work perfectly. I don't see why the desktop environment would matter--the program doesn't have to (and probably shouldn't) care whether I'm on Gnome or KDE or Unity.
Moreover, all the popular environments support both GTk and Qt fairly well, and both are cross-platform, so you would actually be supporting more users than with a Mac OS-only program.
Really, fragmentation like this is not nearly the issue people make it out to be. Basically, everybody can use everybody else's programs on Linux, with the exception of very odd or customized systems. And the sort of people who have very odd and customized systems can fix problems themselves.
EDIT: Also, you could use wxWidgets. That would give you a native look on all the platforms you support.
I would also like to clarify that I understand an OS X user wanting to write an OS X-only program. The real issue is dissuading others from supporting Linux because of non-existent fragmentation issues. They simply do not affect people using GTk or Qt, in practice.
> Moreover, all the popular environments support both GTk and Qt fairly well, and both are cross-platform, so you would actually be supporting more users than with a Mac OS-only program.
By that logic Windows should be first since it has the most users. Also, are you sure that the installed base of Gtk+ or Qt (or both) is greater than the installed base of OS X?
Well yes, because unless I'm much mistaken, Qt and GTk are both supported by OS X so they are both supersets. (This is obviously ignoring Windows, which is also supported by Qt and GTk but is probably difficult for other reasons.)
There are also cross-platfrom native options like wxWidgets. These also work both on Linux and other systems.
Finally, the last StackOverflow survey[1], which I think is pretty representative of the sort of people who would use a program like this, had both Linux and OS X at about the same level (~20% each). So Linux is certainly not negligible.
Moreover, all the popular environments support both GTk and Qt fairly well, and both are cross-platform, so you would actually be supporting more users than with a Mac OS-only program.
Really, fragmentation like this is not nearly the issue people make it out to be. Basically, everybody can use everybody else's programs on Linux, with the exception of very odd or customized systems. And the sort of people who have very odd and customized systems can fix problems themselves.
EDIT: Also, you could use wxWidgets. That would give you a native look on all the platforms you support.
I would also like to clarify that I understand an OS X user wanting to write an OS X-only program. The real issue is dissuading others from supporting Linux because of non-existent fragmentation issues. They simply do not affect people using GTk or Qt, in practice.