Is it necessary to hire a personal trainer to gain weight and muscle in the gym?
It is necessary! But it won't take long, because if you are a thin monkey (I used to be), you'd better buy about 20 private lessons, not more than 30. So at least one private tutor can help you with your scientific training. It is also very important that you will not be lazy if someone forces you to train! This is how I gain 20kg of muscle.