‘ And they did not drive out the Canaanites who dwelt in Gezer, but the Canaanites dwelt in the midst of Ephraim to this day and became servants to do taskwork.'
Compare Joshua 13:13; Joshua 15:63, (although in the latter case Judah ‘could not' drive them out). The suggestion here is that at some stage Gezer was subdued, but that instead of driving them out they made them bondmen, and this was the situation at the time that this was written. This was in direct disobedience to the command of YHWH. This final phrase may, however, refer to a much later time, but the roots of sin began very early. They allowed the Canaanites to dwell among them. Thus the Israelites began to sample Canaanite life, and especially their sexually depraved religion. The Book of Judges will outline what an effect this later had on their faith and obedience.
Gezer was an important city on the road from Jerusalem to Joppa and on the most northern ridge of the Shephelah, overlooking the Aijalon valley, twelve kilometres from the main highway between Egypt and Mesopotamia. It was important for trading purposes. In fact, although the king of Gezer, and its army, were defeated by Joshua, Gezer was not taken. The ageing Pharaoh Merenptah claims to have recaptured it in late 13th century BC, but if so it was only temporary. Archaeology suggests that after 1200 BC the Philistines controlled the city, possibly with Egyptian approval. It definitely became an Israelite possession when the Pharaoh, having seized it, slew the Canaanites and gave it to his daughter on her marriage to Solomon (1 Kings 9:15-17). All this agrees with the fact that Ephraim did not ‘drive them out, allowing them to dwell in their midst'. Possibly like Jebusite Jerusalem for Judah (Joshua 15:63) it was at most times too strong for them. But there would also be other times when if they had exerted themselves they could have achieved it and driven them out. But the impetus was gone and obedience was lacking. They accepted the situation as it was. Once again Israel failed.
This is also a warning to us that we must not settle down in complacency but that by studying the Law of God we should always keep on the alert to do God's will and to drive out all that offends Him.