Weight gain tips?

Lenneth
I have the opposite problem that most people have: I'm too skinny! It's genetic in my family. I am 5' 7" and I am only 105 pounds. 
My dream is to join the military, but I have to be at least 121 pounds to get in 😞
I have tried all of the obvious, like eating so often that I feel like puking, drinking most of my food so that I can eat more calories, and trying to gain muscle. Using these methods I have come up from my previous 103, but it's not fast enough. Anyone have any tips?