Gaining weight needs a steady calorie surplus - eating more energy than you burn. For most men, adding around 300 to 500 kcal a day on top of current intake is a sensible start. Combine it with strength training so more of the gain becomes muscle rather than only fat.
Eat more often, not just bigger plates
If you get full quickly, eat five or six times a day: breakfast, mid-morning snack, lunch, evening snack, dinner and something before bed. Liquid calories such as milk, lassi or a banana-peanut shake are easier to add than another full plate.
A sample day
Breakfast: 3 eggs or paneer bhurji with 2 roti and a glass of milk. Mid-morning: banana with a handful of peanuts. Lunch: rice, dal, a curry with chicken, soya or rajma, sabzi and curd. Evening: peanut butter toast or poha with peanuts, plus a banana shake. Dinner: 3 roti, dal or chicken, sabzi and salad. Before bed: a glass of milk or curd.
Cheap calorie-dense foods
Peanuts, peanut butter, bananas, milk, curd, eggs, soya chunks, chana, rice, oats, jaggery-based laddoos made at home and a little ghee in dal are affordable ways to raise calories. Packaged mass gainers are not required; home food can do the job.
Train and track
Strength train three to four times a week with squats, push-ups, rows and other compound movements, adding reps or weight gradually. Aim for roughly 0.25 to 0.5 kg of gain per week. If you are not gaining after two weeks, add another snack. Unexplained weight loss or low appetite should be checked by a doctor.