1-2: According to Johns prayer Gaius was a faithful Christian who must have been physically ill.
3-4: John's greatest Joy as a teacher was to see the people he taught walking in the truth. The reason for his joy was the confirmation that they were in the faith, removing all doubt of a false conversion.
5: Gaius faithfully showed hospitality to traveling Christian teachers who were often strangers. Word of his love got back to John through the same teachers.
6-8: Through the provision of material resources and hospitality, we can partner with those preaching the Gospel. John says that we do well in supporting those who bring the Gospel throughout the world.
9-10: Diotrephes was a selfish, power-hungry man who's love for power drove him to intercept John's letter and slander the apostles. Seeing the traveling teachers as a threat, Diotrephes excommunicated anyone who welcomed them. John intended to address Diotrephes' behavior in the event of his reunion with the church.
11: As stated in John's other letters, our actions reveal our spiritual condition.
12: John wanted to confirm the good testimony Demitrius had received from everyone.
13-15: John closes this very personal letter in hopes of seeing them in person.