My wife and I are in a mutual gaining relationship. We've been together for over 20+ years. It started off as mainly my fetish. She gained a lot and has been around 300 lbs as of recently. Also recently I am gaining now. With the goal of at least weighing the same as her if not a bit more. Then we can continue gaining together. But, we don't really want to go any more than 350. I too would have been gaining with her all these years. But, because of health issues, I wasn't able to until recently.
Mutual gaining is really great though. Having another person, especially your partner gaining together offers so much. Support, encouragement, trying many things together that a person alone wouldn't be able to do or experience. It also helps with any insecurities. I know many, myself included do have that little bit of us that may worry about what others will say and think. Even if it's just family or friends. Having that person by your side really eases one's mind.
Like any relationship with a kink or a fetish. It may not be an all-the-time thing. For some, it may be. Either way, I will say it's a damn lot of fun.

Best wishes to all.