The apostles didn’t preach the gospel like Christians today
Many Christians today assume that we should tell people “God loves you” or “Jesus died for you” when we evangelize, or else it’s not the gospel. Not saying we shouldn’t, but the apostles didn’t in the book of Acts.
The gospel in the Old Testament: Psalm 2
The second psalm doesn’t get the attention that Psalm 22 and Isaiah 53 get as far as foretelling the gospel of Jesus Christ, but it should, especially since it follows an amazing progression from crucifixion to resurrection to second coming.