Before taking up any form of exercise or diet, do visit a health specialist to determine whether you actually need to gain weight in order to be fit. Just do push ups, pull ups, dips, squats(bodyweight and weighted, jump rope regularly etc. )All the stuff you learnt through your PT teacher in school. That should be sufficient to maintain physical health in most cases, and remedy weight imbalances, if any. You can try gyms as well, but personally don't find it a sustainable way to maintain health and fitness, plus it can get redundant, boring, and can lead to hemmorhoids due to lifting excessive weight, with back and knee problems considering most gyms in India have a bodybuilder type coaching approach. Play a sport if you can. Do some yoga, swimming. Anything sustainable which you can carry on to your 40s and later years.