Gaining

At least a 10% gain, probably a bit more.

There are feedees who gain, decide they don't like it and lose. I haven't run into many of them, but I wouldn't expect to--they're unlikely to show up on sites like this one.

The only advice that can be given that is both ethical and empirical is to not gain much more weight than you intend to keep. How much more varies from person to person--for me it's about 5-10lbs.

Evidence on long term weight loss (2-5 years) indicates that most people who intentionally lose weight end up regaining it, and most re-gainers go on to gain even more. (Studies vary in methodology, conditions/variables addressed, how much weight loss is counted as "successful", how long counts as "long term", etc.)

We still don't fully understand our bodies' mechanisms for long-term homeostatic energy management. We know some of the pieces and interactions (organs, hormones, neurotransmitters, genes, epigenetics), but not the whole picture (though there's really been a research explosion since like 2013 or so--lots of interesting reads).