Xulrunner will be removed in the near future. To support this without the removal of the 'gtk' use flag, conversion to another engine will be necessary and webkit-gtk seems to be the most likely alternative.
gnash nsplugin has already been patched to use npapi-sdk
The xulrunner dependency with USE="gtk" is inherited from before I became maintainer of the ebuild, I am unsure why it is there.
Gnash builds fine without xulrunner now that the patch from bug 383071 has been applied, so I dropped the dependency.