Hello
Go natural, for gaining weight. Creatine can increase your energy levels for heavy workouts. Temporarily you may gain muscle mass but not recommended for long run. It can affect
kidney adversely. So better build muscles by eating healthy foods. Don't look for shortcut.
Milk, curd, paneer, soya nuggets, lentils, pulses, legumes all are good source of protein. If non-vegetarian can have eggs, chicken, fish etc.
Keep doing cardio and workout, and take protein rich diet, you will gain weight.