There is only one "one Gospel" from Adam to Revelation - Gal 1:6-9 and "that gospel was preached to Abraham" Gal 3:8.
Yet as Christ said in John 3 - the Holy Spirit works on everyone - and not everyone is fully aware of it.
Romans 2:13-16 says some folks in following the leading of the Holy Spirit - show the New Covenant work of the Holy Spirit written on the heart and yet "do instinctively" the things of the Word of God - having not been formally informed about it.
Gal.3: 7 Understand, then, that those who have faith are children of Abraham. 8 Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: “
All nations will be blessed through you.” 9 So
those who rely on faith are blessed along with Abraham, the man of faith.
10 For all who rely on the works of the law are under a curse, as it is written: “Cursed is everyone who does not continue to do everything written in the Book of the Law.”[e] 11 Clearly no one who relies on the law is justified before God, because “the righteous will live by faith.”[f] 12
The law is not based on faith; on the contrary, it says, “The person who does these things will live by them.”[g] 13 Christ redeemed us
from the curse of the law by becoming a curse for us, for it is written: “Cursed is everyone who is hung on a pole.”[h] 14
He redeemed us in order that the blessing given to Abraham might come to the Gentiles through Christ Jesus, so that by faith we might receive the promise of the Spirit.
The “Gospel” (Good News) is salvation has come to both Jews and Gentiles, by faith and not by Law. The Law made it easier for us to both see the need to trust (have faith in) God’s love and seek that Love.
That Gospel (Good News) has always been available to people.
The mechanism for seeking and accepting God’s charity might have stayed the same, but Christ going to the cross has been a huge help to both see the huge debt our sins have created, the unbelievable huge Love (a Love which can forgive) and to better realize God as our Parent.