You should be able to gain 1-2lbs (0.450-0.9kg) of muscle at a maximum per month.  If someone tells you that you can gain more in that amount of time, they are either lying or they are using performance-enhancing substances.
When you are developing muscles, you want to build lean, quality muscle.
Building lean muscle takes time as well. So if you want to gain an additional 20kg in size, it is going to take you 2-4 years, depending on the workout routine that you are doing.
So don’t set unrealistic expectations of you building an extra 5 kilograms of muscle in a month. Unfortunately, the body doesn’t work that way.