Which eunuch told the king about the gallows Haman had built for Mordecai?

Answer: Harbonah

Harbonah pointed out the fifty-cubit gallows at Haman's house, and the king declared, "Hang him on it!"

“Then Harbonah, one of the eunuchs attending the king, said: “There is a gallows fifty cubits high at Haman’s house. He had it built for Mordecai, who gave the report that saved the king.” “Hang him on it!” declared the king.”— Esther 7:9 (BSB)