I was in the same boat. Tried eating particular foods but didnt see many gains. Easiest way i found was increasing my meal sizes and eating more meals during the day. Its hard to eat during the whole day especially at work as im eating every 2-3 hours. Pre make your food in containers and then at work just whip them out and munch on.

If you wanna get big you need to eat big. If you aint gaining weight currently increase your meals if it still doesnt work keep increasing.