And when there had been much dispute, Peter rose up and said to them: "Men and brethren, you know that a good while ago God chose among us, that by my mouth the Gentiles should hear the word of the gospel and believe.
When did God do this? Is there any possibility other that Matthew 16:18?
I don't have my Bible with me, but I believe St. Peter was speaking of the events of Acts 10: his rooftop vision in Joppa; the arrival of the servants of Cornelius; his trip to the home of Cornelius; and the subsequent conversion and baptism of Cornelius and those with him.
St. Peter, as the leader of the early Church, was chosen to open the preaching of the Gospel to the Gentiles, a commission which was later taken up by St. Paul.
The council described in Acts 15 occurred well after Peter's vision and subsequent mission to Cornelius.