Gospel to Roman citizens was quite different from gospel of the kingdom in meaning. Both meanings come from the same Greek word, euaggeliou.
To Roman citizens gospel, euaggeliou, “was the news of, or the celebration of, Caesar.” Particularly, “the celebration of the accession, or birth, of a king or emperor.” N. T. Wright
There was the Imperial Cult. The idea of deification of the emperor came during the time of Emperor Augustus. He resisted the Senate’s attempts to name him a god during his reign as he thought himself the son of a god, not a god. Upon his death, the Roman Senate rewarded him with deification which was an honor that would be bestowed upon many of his successors. Often, an emperor would request his predecessor to be deified. Wasson, Donald L. “Roman Religion.” Ancient History Encyclopedia. Ancient History Encyclopedia, 13 Nov 2013. Web. 04 Jul 2017.
What the gospel, euaggeliou, of the kingdom meant and still means to us followers of Christ Jesus is good spiritual news.
Paul boldly declared the gospel of the kingdom that is and is to come. Paul was “an ambassador for a king-in-waiting, establishing cells of people loyal to this new king, and ordering their lives according to his story, his symbols, and his praxis, and their minds according to his truth. This could only be construed as deeply counter-imperial, as subversive to the whole edifice of the Roman Empire; and there is in fact plenty of evidence that Paul intended it to be so construed, and that when he ended up in prison as a result of his work he took it as a sign that he had been doing his job properly.” N. T. Wright
Paul put the gospel this way.
20but our citizenship is in heaven, from which we also eagerly wait for a Savior, the Lord Jesus Christ. 21He will transform the body of our humble condition into the likeness of His glorious [1Pt 5:4] body, [1Co 6:13; 1Jn 3:2] by the power that enables Him to subject everything to Himself. Php 3:20-21 HCSB
The gospel of the kingdom—Jesus
And Jesus went about in all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of disease and all manner of sickness among the people. Mat 4:23 ASV
Jesus established the New Covenant in His blood given for the forgiveness of sins, He went to the cross for the joy that was set before Him (Heb 12:1-3) That joy set before Jesus was His seeing in the Spirit all who repent and believe the gospel joining Him in the New Heaven and New Earth eternally (Rev 21-22).
For now, we worship God our Father and our risen Savior and King through the Holy Spirit. For, God has forgiven our sins, created us in Christ Jesus (Eph 2:10), transferred us into the kingdom of His Son (Col 1:9-14) and poured out His Spirit in us (Acts 2). He has made us His children and heirs and filled us with the Holy Spirit, the earnest of our inheritance, stored up for us in heaven (1 Peter 1:4; 1 Peter 5:1-4). Thus, we worship God in Spirit and in Truth. And, the kingdom of God is in the Spirit (Ro 14:17-18)