Acts picks up where the end of Luke leaves off: on the mount of Olives, near Bethany, where Jesus ascended from.
Some of the people from the places listed below are from gentile locations.
Acts 2:9-11
9Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia,
10Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes,
11Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God.
Acts 2:14
14But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea,
and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:
Acts 2:21
21And it shall come to pass,
that whosoever shall call on the name of the Lord shall be saved.
You do realize that the rest of this chapter that both Jewish and gentile people are present and Peter proceeds to preach the gospel to them, right?
Acts 2:41
41Then they that gladly received his word were baptized: and the same day there were added
unto them about three thousand souls.
Three thousand souls were saved. I'm going to have to utterly dismiss your claim that Acts is useless for doctrine.