What is Gadara?:
Gadara is the site where Jesus exorcised demons from a man and sent them into a herd of swine which then proceeded to jump into the sea. The exact name of the place is a matter of dispute among the gospel authors. Mark says it happened Gerasa, Matthew says it happened Gadara, and Luke says it happened in Gergesa.
Where is Gadara?:
The place where Jesus caused demons to enter the pigs depends on which gospel author is believed. If Mark is right and it was Gerasa, that would be modern Jerash, 50 miles from the Sea of Galilee too far from the sea for the story to work. Gadara may be the ruins of Um Keis, 5 miles from the Sea of Galilee. This was a city of the Decapolis and a plausible candidate. Lukes Gergesa may be the ruins of Khersa, north of Hippos and near steep slopes leading to the shore also plausible.
Why is Gadara important?:
The first act of his ministry among the Jews was casting out a demon; the first act of his ministry among the Gentiles would be to cast out a horde of demons. Jesus is faced with an unknown number, but probably several thousand (a Legion was major unit of the Roman army consisting of four to six thousand men) in one or two men (in Mark we have just one man but in Matthew there are two men).
Because the spirits were unclean, it probably would have been regarded as poetic justice for them to be sent off into unclean animals. Traditionally Christians have read this passage as representing the beginning of the purification of Gentile lands because both unclean animals and unclean spirits were banished to the sea which Jesus had already demonstrated his power and authority over.
Its arguable, though, that this story may have been more properly read as a parable about the unwanted presence of the Roman Legions. They, of course, would not have wanted to be sent out of the country but many Jews would have wanted to see them driven into the sea.