This refers to a region about 35 miles southeast of the Sea of Galilee.
And when he was come to the other side into the country of the Gergesenes, there met him two possessed with devils, coming out of the tombs ... And there was a good way off from them an herd of many swine feeding. So the devils besought him, saying, If thou cast us out, suffer us to go away into the herd of swine. And he said unto them, Go. And when they were come out, they went into the herd of swine: and, behold, the whole herd of swine ran violently down a steep place into the sea, and perished in the waters. Matthew 8:28-32
This refers to the region around the city of Gadara, which is located near the southeast shore of the Sea of Galilee
And they came over unto the other side of the sea, into the country of the Gadarenes. And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit ... He [Jesus] said unto him, Come out of the man, thou unclean spirit. And he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many. ... Now there was there nigh unto the mountains a great herd of swine feeding. And all the devils besought him, saying, Send us into the swine, that we may enter into them. And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand;) and were choked in the sea. Mark 5:1-13
And they arrived at the country of the Gadarenes, which is over against Galilee. And when he went forth to land, there met him out of the city a certain man, which had devils long time, and ware no clothes, neither abode in any house, but in the tombs. ... And Jesus asked him, saying, What is thy name? And he said, Legion: because many devils were entered into him. And they besought him that he would not command them to go out into the deep. And there was there an herd of many swine feeding on the mountain: and they besought him that he would suffer them to enter into them. And he suffered them. Then went the devils out of the man, and entered into the swine: and the herd ran violently down a steep place into the lake, and were choked. Luke 8:26-33
