without flag gtk2 and without gtk2 in system - do not compile DEBUG: configure: error: Library requirements (gtk+-2.0 >= 2.18.0 gtk+-unix-print-2.0 glib-2.0 >= 2.22 gobject-2.0 gdk-x11-2.0) not met; consider adjusting the PKG_CONFIG_PATH environment variable if your libraries are in a nonstandard prefix so pkg-config can find them. depend gtk2 needed... unfortunately..
On my system www-client/firefox-51.0 has been successfully built with the gtk2 flag disabled. But I still have x11-libs/gtk+-2.24.31-r1 installed because it is needed by other applications.
*** This bug has been marked as a duplicate of bug 601326 ***