According to Michael F. Bird,
“The gospel is the announcement that God’s kingdom has come in the life, death, and resurrection of Jesus of Nazareth, the Lord and Messiah, in fulfillment of Israel’s Scriptures. The gospel evokes faith, repentance, and discipleship; its accompanying effects include salvation and the gift of the Holy Spirit.” 
I believe it’s clear that Mike Bird has taken his cue from the Apostle Paul in Romans 1:1-4 and 16-17 in this concise definition of the gospel.
Neither is Mike Bird attempting anything new or revolutionary here (I don’t think he intended it to be). Rather, I believe he simply wants to offer a clear and concise definition of the gospel, and thereby not confusing what the gospel is with what it evokes. In other words, justification by faith is not the gospel but what the gospel evokes.
 Michael F. Bird’s, Evangelical Theology: A Biblical and Systematic Introduction, p. 52.