Jamieson, Fausset and Brown on Amos 1:8

8. Ashdod, &c.--Gath alone is not mentioned of the five chief Philistine cities. It had already been subdued by David; and it, as well as Ashdod, was taken by Uzziah (2Ch 26:6). Gath perhaps had lost its position as one of the five primary cities before Amos uttered this prophecy, whence arose his omission of it. So Zep 2:4, 5. Compare Jr 47:4; Eze 25:16. Subsequently to the subjugation of the Philistines by Uzziah, and then by Hezekiah, they were reduced by Psammetichus of Egypt, Nebuchadnezzar, the Persians, Alexander, and lastly the Asmoneans.