I hear this a lot. I've just never seen anyone who makes this claim actually do it.
Look, regardless of this fact, you don't gain 50lbs of muscle on gear in 3 months, ok?
However, since guys like you won't believe people who use steroids, but instead, believe people who don't use steroids, let's just assume that you actually manage to gain a good ratio of muscle and fat with that amount of bodyweight. How can your body possibly retain this muscle when you've taken it so far away from homeostasis in such a short period of time when you're off cycle? What do you think your body will dump first when it wants to return to equilibrium? It won't be fat.