If I gain by eating lets say more fast food and desserts, is it the same thing as if I were to gain on simply more calories from nuts and cheese? Would the weight go to a different area ? Or is itbetter to gain on healthy things so the body has more 'good stuff' to recover ?
Weight gain is weight gain.
Is one healthier than the other? Why, of course.
^^^ This is correct. Weight gain is weight gain, you won't gain it in a different area, you're body will use all the energy you get from food to recover, but it is much more beneficial for your body to gain on healthy foods too. You should ideally be having a mix of both. Maybe 50% healthy food for the body & 50% of your favourite fast foods and desserts for your mind :) Finding middle balance is key! As long as you need to gain weight, you might as well get the food you've been missing out on! You deserve it!
If you gain from unhealthy foods, your body will not be as healthy because it is not getting as much of the other nutrients, i.e. vitamins and minerals that it needs to repair. Depending on your choices, you might also be getting insufficient protein.
You can still make healthier choices by avoiding the worst fast foods (the ones with no nutritive value whatsoever) and choosing healthier desserts, like desserts with fruit, yogurt or cream cheese, and/or healthy fats.
^^ All of that being said, do what you have to do to gain weight. No foods are "off limits" or "bad." You do want a healthy balance of nutrients (protein, carbs, vitamins, minerals)... but beyond that, have as much fast food and desserts as you want! It's okay. A calorie is a calorie, no matter what kind of food it comes from.