There’s no possible way to gain weight without the right workout regime. Exercise and workout routines need to be specifically in tune with your goals, and if your goal is to build muscle mass naturally, then you need access to volume training. This requires lifting heavy objects, however, we don’t necessarily mean strength training but instead lifting heaving things over and over again. This repeated volume training means lesser weight but higher reps.
Another exercise tip is to focus on using compound movements and exercise during your workouts. These are your squats, deadlifts, bench press, and overhead presses. Find out what your one rep max lift is and use the weight that is around 70% of this with high reps. Aim for six rep sets of one exercise and hit around 8 to 12 reps for each set. If you can do more, then increase the weight. If you’re only pushing out 3 or 5 reps, then lower the weight. This way you are aiming for hypertrophy training where gaining muscle mass is actually the primary goal.