John Chrysostom

And he himself again, along with them, sends up thanksgiving, saying, "Thanks be to God for His unspeakable gift." And here he calls "gift," even those so many good things which are wrought by almsgiving, both to them that receive and them that give; or else, those unspeakable good things which through His advent He gave unto the whole world with great munificence, which one may suspect to be the most probable. …

Homily 20 on 2 Corinthians ↗

Theophylact of Ohrid

He calls "gift" the blessings that come from almsgiving both to those who give and to those who receive. Or: he reminds us of the blessings of which we were deemed worthy through the incarnation of Christ, speaking as if to say: do not think that you are doing something great. The blessings we have received from God are unspeakable. What is so great about our giving what is small and perishable?

Commentary on 2 Corinthians ↗

Thomas Aquinas

From this the Apostle breaks forth in thanksgiving to God, saying: Thanks be to God for his inexpressible gift!, that is, for the charity which is especially seen in you who even help those who thus thank God and pray for you. This gift is inexpressible, because it cannot be said how profitable it is, for "What no eye has seen, nor ear heard, nor the heart of man conceived, what God has prepared for those who love him" (1 Cor. 2:9).

Commentary on 2 Corinthians ↗