Without going scientific on you like some others might, IMO, to gain
lean weight muscle mass you basically have to eat more of what your
eating on a daily basis. Let's say for breakfast if your eating a cup of oats, a tsp of PB, and 3 eggs, than just eat 2 cups of oats, 2 tsp of PB, and 6 eggs.
You don't want to start eating cakes and cookies just to gain weight. Just
eat all the same good body building food but just eat more of it. And at
the gym, lift heavy most days and mix in some light high rep concentrated
days now and than.