5'7 120lbs starting my weight gain journey

I have always been small and underweight. I struggle with a small appetite and a pretty limited palate. I have decided that in the next year I would like to gain 20lbs. I was hitting the gym 5 days a week and eating big for about 2 months this summer and gained 5lbs. I know I can do it, just having a hard time keeping it up. Anyone have advice on gym routing or diet for my body type?