I'm going to tell my gf she needs to at least attempt finishing her meals if I'm paying for them
Does she often leave food behind?
Yes, in my opinion when you pay for her meals you expect her to at least attempt eating it.
I get full sometimes and am I unable to finish my meals, but it's never right to constantly do it and be an appreciative of a meal that someone else have paid for you. There may be an underlying health reason why she is not finishing her meals, so I would recommend that you get her to a doctor and see if she is healthy or if she needs to do something about her problem.
I just want her to finish a meal when I pay for it, who cares if there's medical problems.
If for whatever reason she can't finish her meals maybe you can suggest she gets the rest in a to go bag for later. You can't force someone to eat but at least the food won't go to waste.