16 years old. i go to gym and the trainer is saying to take the mass gainer or weightgainer. will it be alright if i stop taking it after 1 month?

Yes...but: If you are just looking to put on a few pounds I would suggest supplementing with high protein foods and a well balanced diet rather than supplements and weight-gainers. Many of these products contain ingredients and fillers that are not healthy, not labeled, and sometimes dangerous depending on where you buy them. If you are more serious about gaining muscle mass talk to doctor about right ones.