“For Gaza.” The rest of Zephaniah chapter two is about the nations surrounding Israel, starting with the west, then moving east (Moab and Ammon, v. 8), then moving south to Ethiopia (the “Cushites,” v. 12), then moving north (v. 13, Assyria).
“Gaza…Ashkelon…Ashdod…Ekron.” There were five chief Philistine cities that contributed men to a council that ran the government, these four that are mentioned in Zephaniah 2:4, and the city of Gath (cp. Josh. 13:3; 1 Sam. 6:17-18). The reason Gath is missing from this list is unclear, but the most likely explanation is that it ceased to be a city, or a city of any importance, by the time Zephaniah wrote. Gath was attacked and destroyed by the Assyrian king Sargon II in 715 BC, and it disappeared from history after that. In fact, today archaeologists are not completely sure where Gath even was. If it was where archaeologists think it was, it was the easternmost of all the chief cities of the Philistines, which explains why David conquered it, so he would not have any quick surprise attacks from Gath into southwestern Judah (1 Chron. 18:1). However, at some later date, Gath regained its independence from Judah and was considered a Philistine city again, as it was when Sargon conquered it. The prophets who wrote after the Assyrian conquest do not mention Gath when they mention the other chief Philistine cities (cp. Jer. 25:20; Amos 1:6-8; Zeph. 2:4-6; Zech. 9:5).