Welcome to HealthcareMagic. You should continue with the gym routine and concentrate on free weight exercises (which do not require machinery) that target your large muscle groups. The best weight training
exercises for building muscle mass are free weight exercises like squats, dead lifts, bench presses, barbell rows, pull ups and bar dips. You may be having a high metabolic rate, so you need short intense workouts instead of long periods of low-stress
Regarding the diet, eat larger portions and eat at least five meals a day and it should include a balance of carbohydrates, fats and proteins
. You can chalk out a diet chart with your gym trainer or a dietician.
Hope it helps you. Regards.