Seems like it needs the fake X display forcefully, without that it fails to properly identify ruby-gtk and fails. We can either package.use.force it or I don't know if there is a way to force it from the ebuild/eclass.
*** Bug 245750 has been marked as a duplicate of this bug. ***
+ 10 May 2009; Samuli Suominen <ssuominen@gentoo.org> + base/package.use.force: + USE flag X needs to be always enabled in media-sound/rubyripper for Xeconf + wrt #268980.