You will only get one answer from those who actually know what they're talking about. Eat more. That's all there is to it. If you don't gain weight continue eating. If you're like me and don't mind counting calories then do it.
And if you're talking about muscle tissue, yeah, you need to advance in weight in order to grow. And you need food to grow the muscles. I've met a lot of clients who've been like you, what they need to do is face the hard cold facts, they're either;
Not eating enough
Not training hard enough
or a combination of both.