Oh good, so I'm not the only person running into this!
I just submitted https://bugs.funtoo.org/browse/FL-2642if you'd care to wander over and confirm if this is the same issue you're having or toss in more info. I think this is going to require some package/tree magic based on some fairly similar bug conditions (gnome + polkit + spidermonkey version fun) in the closed bug pool. :(
mana got a reaction from keitho2000 in Gnome and Polkit require different dev-lang/spidermonkey versions that can't be installed at the same time