Gabriel said John would go before the Lord in the spirit and power of which prophet?

Answer: Elijah

John would go before the Lord "in the spirit and power of Elijah" to turn the hearts of the fathers to their children.

“And he will go on before the Lord in the spirit and power of Elijah, to turn the hearts of the fathers to their children and the disobedient to the wisdom of the righteous—to make ready a people prepared for the Lord.””— Luke 1:17 (BSB)