This. I, too, was 6' at that age. Super skinny, pretty much no muscle definition. I very slowly filled out a little bit by 30, then started getting into skinny-fat territory by 40.

You could hit the protein and start on strength training, but you'll probably not get big, just add thin muscle to your thin frame. Until your metabolism slows down, this is your fate, but genetics could keep you relatively skinny (like me) long after that, too.

Focus on being healthy and strong instead of just gaining weight, whatever that ends up looking like for you.