Gaining

If she doesn't want to gain any weight, there's no way to convince her to do so without being a shitty person.

I would say don't bring it up so much because what's going to happen is that you're going to work towards undermining the confidence she has in her appearance by constantly telling her that she's not "good enough".

I've had partners who lost weight. It was hard but I eventually adapted. I can't say it will be the same for you, but it might. Hang in there.

You're not a bad person for wanting her to gain, but you may be a bad boyfriend if you keep going on and on about it.