The gospel which Paul preached was taught in the Old Testament
Paul quotes Habakkuk 2:4 to establish the doctrine of Justification by Faith which is the gospel and
which is the theme of Romans
As Paul will show later in the letter, sinners (who have no
righteousness of their own) gain righteousness through faith - not by keeping Gods Law.
By faith alone the unrighteous receive (i.e., are credited with) the
perfect righteousness of Christ (i.e., with the obedience He rendered as a
sinless man)
On the ground of this God-given righteousness believing sinners
are justified (i.e., put in right
standing with God and declared acceptable to Him - Not Guilty! Acquitted!)