The cooperation and love that should be normal among Christians had been broken by the behavior of Diotrephes, who apparently was a leader who abused his position. Evidently Diotrephes regarded other Christian teachers as threats rather than as coworkers; his selfish point of view led him to turn away traveling evangelists and to excommunicate those who welcomed them. See WLC 132, 151. According to Calvin:
Here is a very different example and character - an officer, a minister in the church, who was less generous, catholic [universal], and communicative than the private Christians. Ministers may sometimes be outshone, outdone.
In reference to this minister, we see, first, his name - a Gentile name - Diotrephes, accompanied by an unchristian spirit.
Second, we see his temper and spirit as full of pride and ambition, for he loves to be first. This ferment is an ill, unbecoming character of Christ's ministers - to love to be first, to seek presidency and precedency in the church of God.
Third, we see his contempt of the apostle's authority, letter, and friends...
As to the "gossiping maliciously," Lange says, "They were slanderous words calculated to lower and detract from the apostle's influence, idle, worthless tattle, untenable falsehoods."
Negative Contrast. This is the most critical point of the epistle. John sees a problem in the church (v. 9). It is possible John is worried that Diotrephes will deny what John wrote in his other letters. Diotrephes will lead the church astray if he tells the people to go against John’s leadership. So, John handles this difficult situation here, warning Gaius what lies ahead