We know from different texts of the New Testament that Gaius was baptized of Paul, that he travelled with Paul and was caught in the riot at Ephesus, and that he had since that time often shown great hospitality to Paul. So Gaius is a disciple of Paul through and through. But this epistle is not written by Paul, which I do find to be intriguing.
John wants to focus upon one element of this background as he writes to Gaius. He wants to commend him for his generosity and encourage him to continue to be a fellowhelper to the truth. Due to time, let's start with verses 5-8,
"Beloved, thou doest faithfully whatsoever thou doest to the brethren, and to strangers; Which have borne witness of thy charity before the church: whom if thou bring forward on their journey after a godly sort, thou shalt do well: Because that for his name's sake they went forth, taking nothing of the Gentiles. We therefore ought to receive such, that we might be fellowhelpers to the truth."
So here John makes it clear that Gaius has done faithfully with his giving, not to the church as an institution, but to Christians and unbelievers alike when they are in need. Gaius is helpful and an encouragement to several people.
But his attitude is not universal. John continues by saying in verses 9-11,
"I wrote unto the church: but Diotrephes, who loveth to have the preeminence among them, receiveth us not. Wherefore, if I come, I will remember his deeds which he doeth, prating against us with malicious words: and not content therewith, neither doth he himself receive the brethren, and forbiddeth them that would, and casteth them out of the church. Beloved, follow not that which is evil, but that which is good. He that doeth good is of God: but he that doeth evil hath not seen God."
Diotrephes is the counter-example to Gaius. He has influence and power over the church at Corinth. And instead of using that power to help people, like Gaius did, he uses that power to thwart the help that people give. He is not content with just not providing kindness to those around him, but he will threaten those who attempt to provide generosity.
It is in this context, that John encourages Gaius to continue to follow that which is good and not that which is evil. The easy way out for Gaius would be to succumb to the will of Diotrephes. But John asks that he does not.
John also makes the case that he that does good is of God while he that does evil has not seen God. This verse needs to be understood the way it was written in the Greek. In Greek, verb tense indicates the type of action that is being portrayed. And these verbs are in the present tense, which conveys an ongoing process. The idea is that if one habitually does good then they are of God, while he that habitually does evil has not seen God.
John adds in verse 12 these words,
"Demetrius hath good report of all men, and of the truth itself: yea, and we also bear record; and ye know that our record is true."
This example of Demetrius I think is written so as to encourage Gaius. It may seem that all is lost and that he is the only one who is actually serving God by serving others, but he's not. There is still Demetrius.
It reminds me of the story of Elijah when hunted by Queen Jezebel, when he thought there wasn't a single person serving God alongside him. This is a topic we have looked at before. Yet the Lord assures him,
"Yet I have left me seven thousand in Israel, all the knees which have not bowed unto Baal, and every mouth which hath not kissed him."
So, Christian friend, continue like Gaius and Demetrius in being generous and meeting the needs of those around you.