What did Isaac tell the men of Gerar about Rebekah?

Answer: She is my sister

Isaac said, "She is my sister," because he feared the men would kill him on account of beautiful Rebekah.

“But when the men of that place asked about his wife, he said, “She is my sister.” For he was afraid to say, “She is my wife,” since he thought to himself, “The men of this place will kill me on account of Rebekah, because she is so beautiful.””— Genesis 26:7 (BSB)