Trying to gain weight as someone underweight

Hey everyone! For all my life, I’ve stuggled with weight gain. It seems no matter how much I eat, I don’t seem to gain much weight. I once tried eating so much to the point where the gag reflexes kicked in for a few weeks and I still only gained a few kilos that quickly went away after I missed some days of it. I’m completely new to anything and everything diet-related since I barely even know what carbs and calories mean. I’m getting tested for Marfan Syndrome . If I do have it, I’ll have to stop being very active. I used to go to gym and practice kendo, but I have taken a break now since I’m on a waiting list to get tested for Marfan. Are there any specific diets or food items I should focus on? I’m 180 cm and 57 kg

Trying to gain weight as someone underweight