In Scripture, grace and Gospel are twins. And both are seen together in the life of the incarnated One, Jesus. After all, Jesus, who is the visible personification of God’s grace (John 1:14) and the historical reality of God’s grace (1 John 1:16-18), fully and perfectly embodies both. He is the grace of God, and his finished work is the Gospel of God. Grace and Gospel are sovereignly seen in the Savior.
Ah, there’s the word we’ve been waiting for: Savior. It’s what he does by grace and through the Gospel -- save sinners! For sure, when it comes to our salvation, our Savior, Jesus, is at the center of everything. He is our source and subject. It is his grace. it is his gospel. So a Gospel-centered church is a Christ-focused church. He is the audience -- the eternally worthy One! -- and we are the earthly worshiping ones whom he has sovereignly chosen to save.
Furthermore, the Gospel continues to be the source of all sanctification and growth after salvation. While, as Peter said (2 Peter 1:5), we "add to our faith" by cooperating with the work of God, it is still, nonetheless, the work of God. He started it, continues it, and will finish it, all through the Gospel. As Tullian Tchividjian says, "The Gospel doesn’t just ignite the Christian life; it’s the fuel that keeps the Christians going and growing every day."
Therein lies the essence of a Gospel-centered church/Christian: It's a life that begins and ends every day at the cross (Gal. 6:14).