Gaining

Just some points from personal experience to contribute to your question.
I recall reading about guys who were doing the Channel Island swim, between Ireland and Britain. To complete this, you would need to be incredibly fit, well beyond ordinary fitness levels. However, they would also need an energy reserve, as the swim was so long. So these swimmers were "overweight/obese"-technically speaking. It could be very interesting to read up on their diet/training for someone like yourself OP.
So, from that point, yes, it's definitely possible to be fit and fat.
Also, the last time I ran a half marathon (13miles), there was a guy who was quite fat running at about the same pace as me for the whole thing (9minute miles- not incredibly fast, but you'd definitely need to be fit to do this). I was quite surprised at the time. So there's another bit of, admittedly anecdotal evidence, that fat does not equate to unfit.

An obvious caveat here is that, obviously you can also be fat and unhealthy, someone gaining who had concerns could certainly adapt their diet & lifestyle to accommodate any concerns they would have though

Of course you can be both...Sumo wrestlers are fat and fit.As I understand it,they do the exercise in the morning and then eat huge amounts of food later.
Obviously,processed and takeaway food and sweets and chocolate all the time will be unhealthy,but many oils such as olive oil are good for you and calorific as are nuts and sushi is healthy and loaded with calories

Anyone ever heard of the NFL, WWE or Sumo Wrestling?

Nuff ced

Very interesting thread.
While I only gained a like 10 lbs and am still skinny I am amazed by
a) how easy it is to gain while still burning a lot of calories in exercise
how gaining weight slowly while still exercising has almost no effect on your performance (might vary depending on what you do, for me it's running)

So I guess it's a matter of choice, though I feel some people here seem to think they should not move at all to gain weight.