I heard that a gluten intolerance can make you gain weight but i'm curious if food allergy is related to weight gain? is a myth or not?

Depends on situation: Anyone who must alter their diet to avoid foods they can no longer have can overindulge with the "safe" foods. Weight gain is related to total calorie intake and metabolism. Those with a normal calorie consumption and significant food allergies often loose weight. Untreated gluten sensitivity is often marked by weight loss, not gain.