‘ Beloved, I pray that in all things you may prosper and be in health, even as your soul prospers.'
He knows that Gaius' soul prospers. That is a good testimony to have. He prays that equally his life and health might prosper in every way, that God might ensure that life would treat him well. It may well be that Gaius suffered from physical problems, and that he had these especially in mind. Or John may have had in mind that those who provide for others will find themselves provided for. What a man sows, especially in the name of Christ, he will reap (Galatians 6:7-8)