Gaining

Where you gain is totally genetic. Look at your relatives. If you are thin, but have a little bigger belly, you may gain there. If your butt is larger proportionately, you may gain there.

There is nothing that can change where you gain.

If you look at your pics. And at your height you are still nearly slender. There is a noticeable indentation right above your navel.That means that as you gain your lower belly will become the largest. But you will also grow in the higher part of your belly. When you gain 30-50 pounds more you will have an upper belly laying over your lower belly. It's called a double belly and is the point of Major discussion here on FF. The fact that you are nearly slender and are showing hints of one: It proves that it can be genetic. And Double Bellies are very Attractive.