No, I don't think supplement is necessary, just make sure you consistently eat more than you usually do as gaining weight happens when your body is in caloric surplus. It's better to workout (weight lifting) when you want to gain weight or the weight added is just the weight of fat (instead of muscle which will let you look better).

Keep in mind that the number shown in the InBody test might not be accurate, but it is a good reference. To increase weight, you have to be in caloric surplus state (recommend 200 to 300 more) where daily calories consumption > daily calories burned. Since you are going to gym, I'm assuming you want to add muscle mass so 200-300 calories surplus might do (just for reference) as too much calories surplus will add a lot of fat.

As far as I know, there is an app call MyFitnessPal which helps you to calculate food calories (based on how much gram the food entered by you). The easier way is to eat like usual but then add a meal.

As the InBody's Basal Metabolic Rate might not be accurate, you can weigh yourself once a week to check if your bodyweight is adding. If it's not, then you might have to eat more cause your metabolic rate might be higher than shown by InBody test. Good luck!