Those weight gainers are loaded with a lot of simple sugars that you probably don't need. To gain, think slow and steady. Get 500 extra calories a day from whole foods so your weight gain won't all be fat. That's assuming you know what your total daily caloric expenditure is; figure that out and eat 500 calories above baseline. That will translate to an extra pound per week.
If you feel you must drink your calories, make your own shake with some ground oats, bananas, whey protein, etc. Know what you're putting in your body!