See this account of the demoniacs fully explained in the notes at Matthew 8:28-34.

The Gadarenes - Some of the MSS. have Gergasenes, and some of them Gerasenes. Griesbach seems to prefer the latter. See the note on Matthew 8:28.
The Gadarenes were included within the limits of the Gergasenes. Dr. Lightfoot supposes that, of the two demoniacs mentioned here, one was of Gadara, and consequently a heathen, the other was a Gergesenian, and consequently a Jew; and he thinks that Mark and Luke mention the Gadarene demoniac because his case was a singular one, being the only heathen cured by our Lord, except the daughter of the Syrophoenician woman.

And (1) they came over unto the other side of the sea, into the country of the (a) Gadarenes.
(1) Many hold the virtue of Christ in admiration, and yet they will not lose even the least thing they have in order to redeem it.
(a) See Matthew 8:30

And they came over unto the other side of the sea,.... Of Galilee, or Tiberias;
into the country of the Gadarenes: in the Evangelist Matthew it is called, "the country of the Gergesenes", as it is here in the Arabic and Ethiopic versions. The Vulgate Latin reads, "of the Gerasenes", and so some copies, from Gerasa, a place in the same country; but the Syriac and Persic versions read, "Gadarenes", as do most copies; so called from Gadara, a city either adjacent to, or within the country of the Gergesenes; which was called by both names, from these different places. It was not far from Tiberias, the place from whence this sea has its name, over which Christ and his disciples passed, John 6:1. Chammath was a mile from (e) Tiberias, and this Chammath was so near to the country of Gadara, that it is often called, , "Chammath of Gadara" (f); unless it should be rather rendered, "the hot baths of Gadara": for so it is (g) said, that at Gadara are the hot baths of Syria; which may be the same with the hot baths of Tiberias, so often mentioned in the Jewish writings (h); hence the town of Chammath had its name, which was so near to Tiberias, that it is sometimes reckoned the same with it (i): Pliny (k) places this Gadara in Decapolis, and Ptolemy (l) in Coelo Syria; and Meleager, the collector of epigrams, who is called a Syrian, is said (m) to be a Gadarene, a native of this Gadara. Mention is made of the whirlpool of Gadara (n), which remained ever since the flood. It appears to be an Heathen country, both from its situation, and the manners of the people.
(e) T. Bab. Megilla, fol. 2. 2. (f) T. Hieros. Erubin, fol. 23. 3. & Trumot, fol. 41. 3. & Sabbat, fol. 5. 4. (g) Eunapius in Vita Iamblici, p. 26. (h) T. Bab. Sanhedrin, fol. 108. 1. T. Hieros. Sabbat, fol. 6. 1. (i) T. Bab. Megilia, fol. 6. 1. (k) L. 5. c. 18. (l) L. 5. c. 15. (m) Fabricii Bibliotheca Grace. T. 2. p. 683. (n) T. Bab. Sanhedrin, fol. 108. 1.

Some openly wilful sinners are like this madman. The commands of the law are as chains and fetters, to restrain sinners from their wicked courses; but they break those bands in sunder; and it is an evidence of the power of the devil in them. A legion of soldiers consisted of six thousand men, or more. What multitudes of fallen spirits there must be, and all enemies to God and man, when here was a legion in one poor wretched creature! Many there are that rise up against us. We are not a match for our spiritual enemies, in our own strength; but in the Lord, and in the power of his might, we shall be able to stand against them, though there are legions of them. When the vilest transgressor is delivered by the power of Jesus from the bondage of Satan, he will gladly sit at the feet of his Deliverer, and hear his word, who delivers the wretched slaves of Satan, and numbers them among his saints and servants. When the people found that their swine were lost, they had a dislike to Christ. Long-suffering and mercy may be seen, even in the corrections by which men lose their property while their lives are saved, and warning given them to seek the salvation of their souls. The man joyfully proclaimed what great things Jesus had done for him. All men marvelled, but few followed him. Many who cannot but wonder at the works of Christ, yet do not, as they ought, wonder after him.

And they came over unto the other side of the sea, into the country of the Gadarenes.

They came to the other side. After the storm.
Into the country of the Gerasenes. Matthew in the parallel account says Gadarenes. The explanation is easy. Gerasa, a town whose modern name is Gersa, stood on the southeast shore of the sea, while Gadara, the chief city of the district, was south about seven or eight miles. As the capital city of the district, "the country of the Gadarenes" embraced Gerasa and its vicinity.

Matthew 8:28; Luke 8:26.