the 'top five' are known as such because of the total integrity of the collection of docs. In each of them is all of the 27 NT accounts and letters. That is taken as a better indicator than a great copy of only Paul's letters, or only the non-Paul letters, or what ever sub group it happens to be. As I recall, one good collection has everything but the Rev. Another does not have James and Hebrews.
The collections match lists of docs made by church fathers in the first 3 centuries.
as for the particular language, you can't use Hebrew rules or English rules when it gets down to Greek details. But all languages will diagram the same.
In Gal 2, the target groups were Indirect Objects; the Gospel was the object. It is one. The Greek case system makes that clear.
The word "gospel" and phrase "the gospel" as mentioned in Galatians...
Galatians 1:6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
Galatians 1:7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
Galatians 1:8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
Galatians 1:9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
Galatians 1:11 But I certify you, brethren, that the gospel which was preached of me is not after man.
Galatians 2:2 And I went up by revelation, and communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.
Galatians 2:5 To whom we gave place by subjection, no, not for an hour; that the truth of the gospel might continue with you.
Galatians 2:7 But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter;
Galatians 2:14 But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Peter before them all, If thou, being a Jew, livest after the manner of Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews?
Galatians 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
Galatians 4:13 Ye know how through infirmity of the flesh I preached the gospel unto you at the first.
What was this gospel that 1:7 refers to as "the gospel of Christ"?
Galatians 3:1 says it is "Christ crucified among you..." - "O foolish Galatians, who hath bewitched you, that ye should not obey the truth, before whose eyes Jesus Christ hath been evidently set forth, crucified among you?"
Paul also asserts that he had reminded Peter that said gospel's Grace applied to both himself and Peter...
Galatians 2:15 We who are Jews by nature, and not sinners of the Gentiles, 2:16 Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified. 2:17 But if, while we seek to be justified by Christ, we ourselves also are found sinners, is therefore Christ the minister of sin? God forbid.
In other words, it is the Grace of God - the righteousness of God without the Law - via His Son's Cross...
Galatians 2:20 I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. 2:21 I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain.
Looks to me like one gospel of Christ.
At least as far as this "justified by the faith of Christ" aspect of "the gospel of Christ" is concerned.
Or as Paul had reminded his fellow Jew: Peter - "We who are Jews by nature, and not sinners of the Gentiles, Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified."