How can I gain 6kg in 1 month?

You can’t do it without steroids or other performance-enhancing drugs.

It’s physically impossible to gain that much weight in such a short period of time without using some sort of anabolic steroid or other muscle gaining drug. The average person, even if they were to eat 6000 calories per day (which would be quite difficult to do), would only gain about 2 kg of muscle in a month. So if you want to gain 6 kg of muscle in 1 month, you’re going to need to use some type of performance-enhancing drug.