The author is "the elder" which is almost certainly the Apostle John writing near the end of the first century AD (for fuller explanation, see 1 John Author).
I. Salutation (vv. 1-2)
II. Faithfulness of Gaius Commended (vv. 3-4)
III. Hospitality of Gaius Commended (vv. 5-8)
IV. Selfishness of Diotrephes Rebuked (vv. 9-11)
V. Character of Demetrius Commended (v. 12)
VI. Closing (vv. 13-15)