hm
I've gone through this experience with a couple girls, so I hope my advice to you will be helpful with your relationship.
1.) Like the above posters said, don't hold a double standard. Even if you've got 3% body fat and a 39 bpm resting heart rate, you've got to watch what you eat and how you act around her.
2.) Try picking one of her body parts you particularly like and praise her for it often. Tell her you never want to see that body part change, that it's perfect, etc. She'll instantly start working to maintain it.
3.) If this doesn't work, start pointing out fat girls and talking about how much they gross you out, how their weight is indicative of their self-worth, etc. Hopefully, she'll take the hint and start working out.
4.) PRAISE HER PRAISE HER PRAISE for any work she does to get back in shape. When she passes on dessert, tell her you're proud of her discipline. When she gets back from the gym, take her to bed and ravish her before she gets to the shower. Tell her you like when she comes back from the gym all sweaty. My personal favorite is to say "that's so sexy" when she talks about going to the gym. The point is, if you don't show that you're happy with her efforts to get fit, she probably won't keep working on it. But if you praise her for it, she'll naturally begin to connect her work towards getting fit with your libido. She'll understand your desire to see her fit, and act accordingly.
If none of this works, launch her. I'm very serious about that. Gaining weight can be a sh!t test, she's testing how much you'll put up with before you leave. If you've done all the stuff I've mentioned and she STILL isn't losing weight, you need to send the strong message that you do not accept laziness from girls. If anything, it will build up your reputation for selectivity among the 299,999,999,999 girls in the world.