One time I tried to gain weight just to check if I was able to, as I have been unable to gain any significant weight in a variety of situation (sport, not sport; young, not that young, better/worse diet). I ate a calorie surplus diet (I don't remember the exact numbers, it was a few years ago), which meant eating more than usual but not too much either. This was measuring calories, weighting everything, all the drill; meanwhile I wasn't doing exercise other than walking/biking to places. Again, I don't recall the numbers but I gained maybe 10% of what I was supposed to gain. I've also seen how eating the exact same as other people (sometimes more) results in the other person gaining weight and me losing it, despite the differences in exercise being fairly irrelevant.