This is a personal letter and although it has an audience of one (Gaius), the principles John shares are applicable to all who follow Jesus. The letter encourages the followers of Jesus to provide hospitality and support to traveling missionaries.
Little is known about who Gaius was. His name does appear in isolated passages in the New Testament, all likely referring to the same person.
In addition to the positive example seen in the letter’s recipient, Gaius, Demetrius is also highly commended. Diotrephes, however, is set forth as a negative example, which we are not to follow.
Also, in reading this book, look for the occurrences of the word truth – six times in five of its fifteen verses.