I started the gym 1 month ago and now my trainer told me that you have to take muscletech mass gainer.is the weight gainer have side effects?

Err...: Not sure what side effects it may have as am not familiar with the product, but i would be a little cautious, especially if this is a product that the trainer wants you to purchase from the gym. Most of these otc supplements do little. Do the work, eat healthy food and your body will respond in the way it was designed to. Better to trust nature rather than a man-made short-cut.