If you eat Zone (which is CF recommended) and do the WOD as your main workout, it's going to be difficult to gain weight. You will gain muscle on CF, but my personal experience is that it's not the showy BB type of muscle. My back (esp. lower back) has become very dense and thick since starting CF about 4 years ago. My biceps and chest haven't gotten much bigger at all, though. I'm 42 and don't really care about being big and buff anymore, although I did when I was your age.
As Garrett said, you need to decide what your main goal is and concentrate on that.