What do you suggest i do to gain muscle mass in the gym?

Consistency: In a proper weight lifting program: choose a frequency you can maintain - at least twice a week per muscle group, but not more often than every other day; good technique and 3-4 sets of 6-12 reps with full effort. Good hydration, and good diet with enough protein. Do not overdue or injury will prevent progress. Ask/watch the big guys in the gym for the exercises -- they'll tell you.