Gaining

I'd say incorporate it more into your love life (if this is with a SO), and make it more of a game than a chore.

Two ideas I have:

1. Incorporate food into your sex life to help spice things up and help both of you enjoy it more.

2. Take him/her out often to eat if you can as romantic dates.

That's all I've got/can think of at the moment. But yeah, I could imagine you just have to make it fun for the both of you rather than having it as a one-sided thing, and your feedee will definitely put on the weight ^^.

After typing this all out, it seems like I'm not really adding anything to what people already know now that I'm thinking about it. Oh well!

Take her shopping and shop for her ... she should always have clothes that fit well (you might prefer to see her in too tight clothes, but that isn't a good every day plan).

Massage her, as gaining weight can be tough on back and legs). And encourage her to stretch and stay flexible.

I disagree about encouraging inactivity -- over the long run staying active pays off. But you can help her find activities that work with new inches. Walking or biking instead of jogging, etc.