I have never seen Gale in another role. In fact, up until recently, I denied there was a Gale. Cause you know, Brian is a real person. So I will answer in terms of Brian. snort.
Sooooo, I think it is something Brian does when he is feeling vulnerable or trying to come off as vulnerable. There are times he seems to do it unconsciously, others when he seems to be saying "Look how damn cute I am when I do this, how can you resist me?"
And I have to answer, you do look damn cute when you do it. As with the gum chewing.
Not a deep answer, I'm wading in the shallow end of the pool tonight, I guess. :)