There is conservation of energy, you're not going to gain mass without eating, but if you change nothing else and take a myostatin inhibitor or any other medication that inhibits myostatin (like growth hormones) it is guaranteed you will gain muscle.

There are many people and animals who have dysfunctional myostatin so we already know what happens when myostation is inhibited.

Growth hormones inhibit myostatin so any regular GH or GHRP will help build muscle and retain it and the effect of these have been extensively studied.