Is weight gainers has side effects?

Well...weight gain. Usual weight gain supplements are just a concentrated form of calories - carbohydrates, protein and fat. Some have upwards of 1000 calories per shake - some even more. So the issue becomes weight gain, not the good kind, if you're not using those extra calories. If your activity level does not counterbalance the increase in calories, you'll gain weight, but not healthy weight.
Gaining weight. High calorie foods can help you gain weight, but the side effects are only if you gain too much.