By "knee gap" I assume you mean osteoarthritis of the knee. This is an ageing process which cannot be prevented. However few things are known to delay the onset and progression of the problem. Walking is good for you ; running, jumping, skipping are not. Cycling and swimming are good too. Do exercises to strengthen your thigh and hamstring muscles, they reduce the load on the joint. Avoid sitting cross-legged and use only the western type of toilet. Lastly consult an orthopedic surgeon if you have pain and stiffness in other joints too - this is usually not osteoarthritis.