In short
The roughly two thousand pigs (Mark 5:13) belonged to the non-Jewish Gadarenes, because Jews viewed pigs as unclean animals (Leviticus 11:7).
This was a non-Jewish area. The Jews viewed pigs as unclean animals (Leviticus 11:7). As such they also regarded swine husbandry as an illicit profession. This is why there were pigs in the land of the Gadarenes, as the shepherds of this herd of pigs would have been non-Jews. According to Mark 5:13, the herd consisted of about two thousand animals.1
30 Now a herd of many pigs was feeding at some distance from them.