I love the excuses for failure, when the plan is easy. If you eat more calories than you consume, you WILL gain weight, as sure as the sun will rise in the morning.
In my case, I weighed 175 at 12% at 17 at 5'5". In the case of flex89, he gained 74 pounds of muscle, in five months, and lowered his fat by 2% by doing HIIT, and lifting high reps, without any magic pills, all while being a type 1 diabetic, at 19, and 5'10", at 208. flex89 eats a minimum of six times per day (good clean stuff like chicken, grits, and veggies), following my diet and training methods. I've helped numerous young men (without the fragile egos of the poster) achieve their training goals, time and again.
I haven't been as thin as 120 since I was in grade school. I grew up eating good.
Everyone has gotten off track, trying to be politically correct. The poster's question was how to gain weight. The poster did not stipulate that he would not drink weight gainer, or that he wouldn't eat carbs. The poster said he was tired of being called "tiny" but at 5'9" and 120# that's an adept description of his body. The poster took great exception to the fact that if he is in fact eating, then he must surely be physically ill, mentally ill, or be more active than what he's eating.
All the drama and I,I,I,me,me,me, victim mentality of the poster aside, the fact remains that if he does not have a physical illness, mental illness, he WILL, very much IN FACT, gain weight if he consumes more calories than he expends. I didn't stipulate where those calories would come from, but, went to the effort of showing the poster how to monitor, and compute those calories, and what he might do to make sure he was in a good state of health.
Many folks who are very thin have metabolic disorders caused by hyper-active thyroid, or they're anorexic, and undiagnosed.
The advice I gave was sound, with a plan for success. The poster wasted our efforts by continuing to sabotage his own, saying he wanted to eat steak. Now, he could eat steak and potatoes several times a day, and, if he monitors his calories he WILL gain weight.
In a word, to be successful, there's no special pill, nor potion: EAT.
The poster is what a trainer or coach would call un-coachable, sabotaging good sound coaching, and continuing to complain about failure, and being very insecure at every corner. No one has said the poster is ill. He needs to eat, and, he needs to understand why he feels he needs to lash out at others who take the time to help him achieve his goal, and why he can't understand why successful folks are successful (why he resents that).