What did the Gibeonites become for Israel after their trick was discovered?

Answer: Woodcutters and water carriers

Because of the oath, Israel let the Gibeonites live but made them woodcutters and water carriers for the congregation and the altar.

“They continued, “Let them live, but let them be woodcutters and water carriers for the whole congregation.” So the leaders kept their promise. On that day he made them woodcutters and water carriers, as they are to this day for the congregation of the LORD and for the altar at the place He would choose.”— Joshua 9:21, 9:27 (BSB)