Want to gain 20 kg by August I'm 5"11 and 172lbs.

You are currently 172lbs, and you would like to be 216lbs by August? (Sorry, confused by use of both kg and lbs.) That seems a little fast, but I'll let people with more knowledge weigh in on that (especially since you're young; you may do better at packing on weight).

You say you're tired of eating a lot and not gaining. That would likely mean you aren't eating enough yet. What are your maintenance calories, what is your bulking goal, what kind of foods do you eat to get there (nutrient dense foods will require a very large volume of food, which may make it hard to hit your goal), what is your training program like?

And go ahead and give this a read:
http://community.myfitnesspal.com/en/discussion/10049766/bulking-a-complete-guide-for-beginners

Not a problem at all. How much of that do you want to be lean mass?

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