How do we know the gospel is foreshadowed in the Old Testament?

Historically, the Old Testament serves as a foundation for the New Testament gospel, implying that the entirety of Scripture is about Christ. The Apostle Paul, as mentioned in Galatians 4, interprets Old Testament accounts such as that of Ishmael and Isaac as allegories illustrating the distinction between law and promise. The fulfillment of these promises is seen in Jesus, who embodies the gospel narrative, demonstrating that the Old Testament's sacrificial system and prophecies lead to Christ's ultimate sacrifice. Hence, understanding the Old Testament through the lens of the gospel highlights its purpose of revealing Christ's work for our redemption.