The Ephraimites did not conquer the Canaanites living in Gezer. The Canaanites live among the Ephraimites to this very day and do hard labor as their servants.

Whenever the Israelites were strong militarily, they forced the Canaanites to do hard labor, but they never totally conquered them.

I gave you a land for which you did not labor, and cities which you did not build, and you live in them; you eat from vineyards and olive groves which you did not plant.’

And they returned to Joshua and said to him, Let not all the people go up. But let about two or three thousand men go up and strike Ai. Do not make all the people labor there, for they are few.