"Now if a man works his wages are not counted as a gift but as a fair reward. But if a man, irrespective of his work, has faith in him who justifies the sinful, then that man’s faith is counted as righteousness, and that is the gift of God." Romans 4:4,5
This is so important - let's extrapolate it out a bit:
There is a God: the I AM.
His very living essence is goodness, love: righteousness.
When a man or woman - fallen, fleshly, broken and sinful by their very definition - begins to believe in God, they do something. They align their life with the goodness, love, the righteousness, of God, and they begin a different journey through their human existence.
When they believe in God, when they trust in God's ways and means, they begin choosing against all forms of self-righteousness. It is not their works - the sweat of their spiritual brow - that will justify them: it is the gift of God who, by belief, lines up His Way (His goodness, love: His righteousness) with this new, redeemed human being.
The whole thing is perfectly natural and perfectly supernatural.
It is "the gift of God" to the individual who, simply, believes.