In the notes in the New American Bible, Gerasene (also called Gadarenes) is a pagan territory across the sea (of Galilee, according to Luke); this explains the presence of pigs. As well, that Gerasene is a pagan territory near Jewish cities, indicates that the person (or persons, if we'll take Matthew's account) saved was a Gentile.
Of course, this narrative, like many other biblical narratives, may be more theological than historical; it is evident that even the synoptic gospels do not entirely agree on the facts in this passage.
Perhaps the message behind the travelogue (in which this narrative is included) is that the authority of Jesus as the Son of God is more powerful than:
Source:
New American Bible, The New Catholic Version