It totally depends on your goal and what you want to achive. Both has there own benfits; Gym- provided you will do weight exercise will build muscle mass while shading off fat and muscle is a crucial part of the human body that will fight off aging and enhances strength also immunity. Yoga- Will enhance flexibility, increase blood flow and also mind body connection so it also has its own benfits. But if i could suggest I would do both 4 days aweek gym and three days aweak yoga since they both compliment one another