The first mention of the gospel was in Genesis 3:15. It is referred to as the Protoevangelium, that is the gospel before Christ came. Here we see that Christ is the Seed of the woman. His defeat of the evil one who tempted our first parents is announced as sure.
We see the gospel promise given to Abraham through covenant in Genesis 12:1-3. God gave him this promise: "In you all the nations shall be blessed." That blessing comes through Abraham's Seed who is Christ (Galatians 3:16).
Old Testament believers had the gospel in Genesis of the Messiah who was coming. As believers today, we know that He has come "that the blessing of Abraham might come upon the Gentiles in Christ Jesus, that we might receive the promise of the Spirit through faith" (Galatians 3:14).
All believers receive the blessing as Abraham did in Genesis. He was justified before God by faith. "And he believed in the LORD, and He accounted it unto him for righteousness" (Genesis 15:6). The gospel has always been received by faith (Hebrews 11:39-40).
The gospel came to Abraham in Genesis through the covenant promise. The gospel comes today to all believers through the covenant fulfillment. "Jesus Christ is the same yesterday, today, and forever" (Hebrews 13:8).