Back in Antioch after their first journey, Paul and Barnabas reported God had opened what to the Gentiles?

Answer: The door of faith

They gathered the church and reported all God had done through them and how He had opened the door of faith to the Gentiles.

“From Attalia they sailed to Antioch, where they had been commended to the grace of God for the work they had just completed. When they arrived, they gathered the church together and reported all that God had done through them, and how He had opened the door of faith to the Gentiles.”— Acts 14:26-27 (BSB)