Having received grace from God, can we merit the gift of more grace?
Augustine says (super Ep. Joan.; cf. Ep. clxxxvi) that "charity merits increase, and being increased merits to be perfected." Hence the increase of grace or charity falls under merit. [ST I-II, Q114, A8]
As always, though, Aquinas is careful as to what exactly he means.
As stated above (6,7), whatever the motion of grace reaches to, falls under condign merit. Now the motion of a mover extends not merely to the last term of the movement, but to the whole progress of the movement. But the term of the movement of grace is eternal life; and progress in this movement is by the increase of charity or grace according to Proverbs 4:18: "But the path of the just as a shining light, goeth forward and increaseth even to perfect day," which is the day of glory. And thus the increase of grace falls under condign merit.
Now as we saw, condign merit comes from grace; indeed, all merit flows from God's grace, which is why (as we've seen) Augustine says that God rewards that which he has given. Consequently we see that the Catholic gospel isn't works-based, contrary to the errors spread by some; rather, the Gospel is based upon grace.