Gaining

I've heard time and time again that consuming highly calorific foods before bed speeds up the gaining process, since the body converts all the calories into fat as you sleep.

Does this really produce overnight results? And where is the fat likely to go?

I could not imagine that it produces "overnight results". It certainly, though, helps. Much weight gain is based on intake of calories and the amount of exertion. I recommend eating a meal before bed because it's taking in a lot of calories while doing nothing (sleeping), and also because I've noticed in the little husband for example, that over time it helps the body slow down, gets a person accustomed to little exertion, and helps establish the feeling of being full and doing nothing.
So, over time yes, over night, not really.