The answer
The word "gospel" is a Greek word meaning good news.
St. Paul the Apostle used it in this sense, not meaning a certain book. In some instances he said, "The gospel of your salvation." (Eph 1:3), i.e. the good news of your salvation. In other instances he said, "The gospel of peace." (Eph 6:15) meaning the good news of peace or preaching peace and "The gospel of the glory of Christ." (2 Cor 4:4) and "The glorious
gospel of the Blessed God." (1 Tim 1:11) by which he means preaching about this glory.
Of course, there were no gospels carrying these or other names.
When St. Paul the Apostle said, "The gospel for the uncircumcised had been committed to me, as the gospel for the circumcised was to Peter." (Gal 2:7), he meant that he was entrusted to carry the gospel or the good news to the uncircumcised, i.e. the Gentiles and St. Peter to carry the gospel to the circumcised, i.e. to the Jews.
What is meant by gospel is the good news of salvation and redemption. He did not mean of course that there was a gospel called gospel for the uncircumcised and another called gospel for the circumcised.
The same is understood from all other words of the Apostle.
By the words, "My chains for the gospel," (Philem 13), he meant the imprisonment he undergoes for his preaching the gospel. And when he said, "The things which happened to me have actually turned out for the furtherance of the gospel." (Philem 1:12), he meant the furtherance of the preaching of salvation. By the words, "I have begotten you through the gospel." (1 Cor 4:15), he meant the preaching he preached. The same goes for all other texts because there were no written gospels at that time.
The Lord Christ Himself used the same expression.
At the beginning of His preaching - when John the Baptist was in prison - the Lord Christ came "preaching the gospel of the Kingdom of God and saying, 'The time is fulfilled and the Kingdom of God is at hand. Repent and believe in the gospel'" (Mark 1:14, 15).
Which gospel was it that the Lord Christ meant, though there were no written gospels at that time and He had not yet chosen His disciples?
He meant then to say "Believe in this preaching of the Kingdom which I preach you now."
It is the joyful news that the Kingdom of God is at hand.
Christianity came preaching salvation; salvation from the punishment of sin and of the dominion of the devil, eternal salvation through redemption. This preaching was given the name "gospel".
The same can be traced in the Lord Christ's words where He used the term "gospel" often. An example of this is found in the words of the Lord to His disciples, "go into all the world and preach the gospel to every creature." (Mark 16:15).
There was no written gospel at that time, but the Lord Christ meant preaching the news of salvation to all people.
The same applies to St. Paul the Apostle; by the words, "The gospel which was preached by me," he meant the good news of salvation which he preached.
Moreover, "I went up again to Jerusalem..... and communicated to them the gospel which I preach among the Gentiles" (Gal 2:1, 2); by which words he meant the preaching among the Gentiles that they also have attained salvation.
When he said, "For God is my witness, Whom I serve with my spirit in the gospel of His Son" (Rom 1:9), he meant preaching about His Son; for there is nothing called "the gospel of His Son" or "the gospel of Christ".
Thrice-Blessed Pope Shenouda III of Blessed Memory