General

If you're comfortable with it I'd share it with her. Communication and honesty are extremely important for a successful relationship. Slowly introduce her to it and how you feel about it all. Sharing all that with her doesn't mean she has to gain or anything. This fetish isn't for everyone. She may tell you she is fine with it and may do some gaining. She may naturally gain thru life for various reasons. Many people keep this fetish in the realm of fantasy. Which you can also do. It may not seem as fulfilling, but for some it is. You won't know how she feels about it all until you talk to her. And tell her you love her no matter what and that she doesn't have to gain if she doesn't want to (of course). Then take it from there and see how life goes.
I was afraid to tell my wife, girlfriend at the time. While she isn't a feedee and was also skinny. She just ended up letting herself go, and ended up actually liking her size now and how much I love how she is now too. Of course I love her no matter what too. But it comes down to that honesty and communication. Best of luck

I would say bring up the fetish again and talk about what interests you about it. Tell her your concerns about how you feel having the fetish will affect your relationship long-term. From there you could ask to figure out an outlet together for the fetish if need be.