John Chrysostom

"Gaius mine host, and of the whole Church, saluteth you." See what a crown he has framed for him by bearing witness to such great hospitality in him, and brought in the entire Church into this man's house! For by the word used here, he means a host, not a guest. But when you hear that he was Paul's host, do not admire him for his munificence only, but also for his strictness of life. …

Homily on Romans 32 ↗

Theophylact of Ohrid

That is, the one who received me in his house. Furthermore, that Gaius received in his home the whole Church and Paul himself, this is great praise; for Paul would not have gone to him if he had not found him worthy. The steward and treasurer of the city of Corinth. He mentions him so that you may learn that neither wealth nor rank serves as a hindrance to anyone in faith and righteous living.

Commentary on Romans ↗

Thomas Aquinas

Then he says: Caius, my host, was the person to whom John wrote in his third letter commending the charity he exercised toward the saints; and the whole church, namely, who were assembled in his house or who were in that region. Then he says: Erastus, treasurer of the city, i.e., who guarded the city's money, and Quartus, a brother.

Commentary on Romans ↗