The Apostle is writing to his friend Gaius. Whom he declares walks in truth. Further he describes Gaius as being generous, faithful in caring for others, loving the church, supporting the brethren and encouraging others in their missionary work. Wow, what a commendation.
Gaius is contrasted with Diotrephes whom The Apostle describes as loving to have preeminence in the church and refuses to receive the brethren or provide them with shelter, food or other necessities. Diotrephes also forbids others from the receiving the brethren (visiting missionaries) by putting them out of the church. The Apostle declares this to be evil.
Gaius is cautioned to practice only those deeds that are good. For he who does good is of God (11)
Demetrius is another person mentioned in this epistle. Demetrius has a good testimony to all.
John ends this letter with the hope that he will see his friend soon face to face.
My takeaway is to walk in truth and practice those things that are good and not evil.