The ebuild translates the USE flag "gnome" to "ENABLE_THUMBNAILER". Now when we look at ffmpegthumbnailer this flag has nothing to do with gnome but places a file at /usr/share/thumbnailers to use by other DEs (I'm using it just fine with LXQT while writing this). See https://github.com/dirkvdb/ffmpegthumbnailer/blob/master/CMakeLists.txt#L190 for more infos. So the USE flag "gnome" should not depend on the USE flag "gtk" and it should be renamed as it's completely independend from both, gnome and gtk.
The « gnome » useflag is really a misnommer (and its description too, but that's logical). We spent one day to understand why videos where not thumbnailed despite « everything being properly installed », on a non-really-Gnome system (Awesome WM, caja file manager, -gnome useflag by default). Luckily, this bug report led us to the answer. The useflag should be renamed into something clearer.
