The word gospel means good news, but different passages have different aspects of the good news in mind from place to place.
1. There is the good news that "Jesus is Lord."
It is the gospel concerning God's Son, of the seed of King David (Rom. 1:3).
2. There is the good news of his coming kingdom.
Jesus preached that the kingdom was near, to repent and believe in this good news. (Mark 1:14-15)
3. There is the good news of Jesus' death and resurrection. (1 Cor. 15:1-8)
On the basis of this good news, we can be saved.
4. There is Jesus' good news to the poor, to captives, to the blind and oppressed. (Luke 4:18-19)
So there you have it. A gospel church is one that preaches that Jesus is Lord, that he died for our sins and rose victoriously from the grave. A gospel church preaches good news to the poor and marginalized now, and looks forward to the restoration of all things in the coming kingdom. A gospel church looks forward to the salvation that will take place when his kingdom fully comes.