Tk *does* look natively on Windows, when used wisely: http://tktable.sourceforge.net/tile/screenshots/windowsxp.html

More to that, you can have windows look-n-feel on linux, and many different similar things.

As for Gtk2, I was disappointed by its usage on Windows: quite old binaries, which are hard to re-build (many dependencies on different GNU libraries).
Also, Gtk2 binaries on Windows occupy 18Mbytes on my system, and, while widgets are interesting to see, they are not native look-n-feel ...

At least Tcl/Tk is much much more ligthweight than Gtk2, easier to setup, rebuld, develop/distribute software and use.