Did they know that God would turn to the Gentiles and call out the Body of Christ?
No, the Twelve and Peter did not know God would turn to the Gentiles and call out the Body of Christ. The context indicates God had not yet revealed this; they could not believe what He had not spoken. The revelation of the Body of Christ came later, not during Acts 3.
Les teaches that Peter and the Eleven did not know God was going to turn to the Gentiles and call out a people for His name, the Body of Christ. He explains that at Acts 3 God had not yet said a word about shutting down the temple or moving from law to grace. They were not negligent; they simply could not believe what God had not yet spoken. As Deuteronomy 29:29 says, secret things belong to the Lord, but revealed things belong to us. Faith comes by hearing the Word of God, so they had to wait until God spoke it. At that point, their gospel was still based on who Jesus was—the Christ, the Son of the Living God—not on death, burial, and resurrection for the Body of Christ.