Are you sure you want to gain? If you eat properly and get plenty of exercise, you should be okay. Otherwise . . . there is no otherwise. Figure it out.
One thing to keep in mind . . . some people in Mongolia live to 120 on a diet of meat, mare's milk, mare's cheese, and mare's curds. There are very few vegetables, or refined grains in their diet.
That won't work for me. I crave fish, and on occasion . . . bangers and mash . . . samosa . . . enchilada . . . mole' . . . el ropa . . . roti . . . cous cous . . . chicken bog . . . big fat Texas T-Bone . . . Poy! . . . Gator Tail . . . lean pork chops, oven fried . . . sweet 'taters . . . beer . . . Jamaican Patty . . . fried dumplings . . . pressed duck . . . all kinds of pie . . . sorry, I am going to get a mega-blaster burrito with seared tuna and lobster!