Rom. 10:16 But they have not all obeyed the gospel. For Isaiah saith, Lord, who hath believed our report?
Multiple Scriptures testify to the fact that the Gospel existed before the coming of the expected Messiah.
Gal. 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.
9 So then they which be of faith are blessed with faithful Abraham.
And, from Hebrews,
Heb. 4:2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.