This is something I've always scratched my head about. I honestly have never gained something and then lost it. I think the guys who make "gains" and then lose them, actually had "gains" confused with swelling, or a pump. I know for a fact that if I jelq 5 or 6 days a week...I will measure a little bigger, but then if I take off, of course the swelling goes down. There's no "loss" happening though.
I have always just done my exercises with the thought in mind that if I dont stop and have a sound program, I WILL gain. I keep at it until I see a gain.