The Holy Spirit.
So the Catholic Ephesians 2:8 could read:
8 For by grace the Holy Spirit you have been saved through faith; and that not of yourselves, it is the gift of God;
and the evangelical version:
8 For by grace the generosity of God you have been saved through faith; and that not of yourselves, it is the gift of God;
I can’t speak to the accuracy of the translation of this verse, but yes, the Catholic definition of grace is the “Life of God” or the “Holy Spirit” in the soul (specifically, “sanctifying grace.” Actual grace is different.)
The fundamental, conceptual, Catholic, doctrinal distinction is that God changes us intrinsically, making us truly holy.
The “imputation” doctrine is extrinsic. God overlooks our intrinsic sinfulness, “covering” our sins by Christ’s atoning sacrifice.
If my understanding is correct, this doctrine contradicts God’s nature as Truth and Justice.
Moreover, this doctrine contradicts scripture, which tells us that no sinner can enter heaven.
Finally, the fact that grace is “the Life of God” or the Holy Spirit in our souls, does not preclude the gratuitousness of grace and salvation.