John Gill Commentary 3 John 1:4

"Greater joy have I none than this, to hear of my children walking in the truth." — 3 John 1:4 (ASV)

I have no greater joy
Nothing that causes greater joy. The Vulgate Latin version reads "grace" or "thanks"; and then the sense is, that he had nothing to be more thankful for:

than to hear that my children walk in truth ;
meaning his spiritual children, those whose conversion he had been the instrument of; and among these it seems Gaius was one.