Concerning gravity, it does two things basically; it comes as an attractive force described by the Ricci tensor, but also as a tidal force described by the Weyl curvature tensor, that distorts objects due to the gradient of attraction: when standing upright your feet are feeling a greater pull from the Earth's gravity than your head, literally pulling you apart, or trying. In a black hole you'd be what physicists call spaghettified. I would certainly imagine that playing with that gradient could induce sensations one would find hard to classify.
If gravity (or mass) went away altogether, you'd behave like beams of light - or rather, the matter that used to be your body would. If it was substantially greater, you'd undergo spontaneous gravitational collapse. Play with gravity and you can do some really eery sh*t. (Roger Penrose; The Road to Reality and other works; Victor J. Stenger; The Fallacy of Fine Tuning; just to name two)
But the question is of course whether you want to go that way. You'd probably have to find some elegant way of introducing some of these concepts, so that in the crucial moment - if it is a crucial moment - we'd understand perfectly why she couldn't even tell what her sensations were.
And on a personal note; as one afflicted with recurring depressions since childhood I can attest that there are sensations you absolutely can't even tell what are; and experiencing that you live in a different universe from the rest of humanity is a fairly accurate way of describing it. Of course that doesn't mean your work has to be about depression, though the experience of your protagonist likely would induce something along those lines. Hyperbole and a Half recently did a piece on this kind of alienation: http://hyperboleandahalf.blogspot.dk/
Well, I might have veered off topic somewhat... I don't see anything wrong with the greenish face. It works for me, though I might have substituted galaxy for civilization. That it's Ruth seeing her like that seems to me implied by the context. You may want to follow up with something like "Ruth had a sudden premonition that..." just to make it obvious, and maybe also to expand the metaphor; let it run for a while. You've got a good thing there. Run with it. One does not have to state explicitly who says or thinks what all the time, and in imho that might actually ruin an otherwise elegant piece.
"She did all this mechanically...": I'd get rid of as if. She's a waitress, employed in a menial, routinely task; she probably is running on autopilot. And although your protagonist can't strictly know that, she may certainly think that, and not even think it was anything out of the ordinary. Now, if the waitress was not on autopilot; if serving this particular beer for that particular customer really meant something; that would be odd. Maybe she was hitting on her. Maybe she thought your protagonist was the reincarnation of an ancient Mayan goddess or something. Maybe not. Maybe she was just a waitress on autopilot. And I know you did not solicit this piece of advice, but I like to let my characters daydream, imagine contrafactual scenarios; it adds texture to their character plus it's a great way of expanding on a metaphor.
On the whole, the best metaphors are those that take on a life of their own as recurring motifs; the green face, eerie light, her intrinsic alienness, noticing other people running on autopilot. Those could all work. But odd gravity is a bit tricky, as it may require people to know things one cannot readily assume they will know. If it was my story, that would not be a metaphor; that would be the reality that requires some metaphor, that people, including the characters, may understand it eventually.