With regard to weight gain, please do make sure that you have healthy weight gain and not any unhealthy version.
Here we find that people look at weight gain and associate it with prosperity. However world over the cultural norms are different, as long as people are not anorexic....
Healthy fat, protein, wholewheat, lots of greens are all good.
Now about fitness, for a good session, we need warm up, exercises that involve strengthening, stretching, agility/balance and cool down. For this *you need to have a physiotherapist consultation.
Next Steps
Fitness could be achieved with exercises, but you need a physiotherapist consultation. This is necessary as certain exercises could be too streineous and hurt you. So there needs to be supervision and a good progression plan could be tailormade to your need.
Health Tips
Also take a dietician appointment for dietary side of things