During the time of Jesus, much of Palestine was under Roman control as part of Syria. At that time, Syria could mean the whole eastern Mediterranean coast. Matthew mentions Syria to show how far news of Jesus spread.
those having seizures: This refers to epilepsy or demonic possession. In 17:15, a person described this way "falls into the fire for into the water," meaning the boy could not control himself.
The Decapolis was a group of ten cities, mostly east of the Jordan River and southeast of the Sea of Galilee. These included:
Damascus,
Raphana,
Hippos,
Abila (or Canatha),
Gadara,
Scythopolis,
Pella,
Dion,
Gerasa, and
Philadelphia (Amman).
These cities were mostly Gentile (non-Jewish) and followed Greek culture.