Overview
Gaius was a common name and occurs 4 times in the New Testament. We do not know who this particular one was but he was probably a friend of John with some responsibility in the church.
In this very short book John encourages Christian fellowship. The book contrasts four different church men:-
Author
The apostle John in Ephesus, probably delivered by Demetrius
Time of 3rd John
Very late in Johns life. Probably around AD 90