The cities of Gaza and Ashkelon represented what was left of the Philistines' strength.1 Baldness
probably refers to a time of mourning because of defeat. Ashkelon has perished as well. The question about how long the Philistines will “gash" themselves could either be a reference to mourning or rituals that form part of their idolatry, or both. Either way there is no respite from the defeat they face.
5 Baldness has come upon Gaza; Ashkelon has perished. O remnant of their valley, how long will you gash yourselves?