3 John 5 - 8

5 Beloved, you do faithfully whatever you do for the brethren and for strangers, 6 who have borne witness of your love before the church. If you send them forward on their journey in a manner worthy of God, you will do well, 7 because they went forth for His name’s sake, taking nothing from the Gentiles. 8 We therefore ought to receive such, that we may become fellow workers for the truth.

Gaius showed by his actions that his works were done full hearted as expected by God. (Colossians 3:23) He did it as if what he was doing was done to the Lord and not unto men. Christ spoke of our rewards or punishments as what we did for Him or not done for Him. (Matthew 25) Gaius did his good to his brothers and sisters in Christ as well as to those who are in the world. (Galatians 6:10)

Traveling brothers and sisters in the church testified to others how Gaius did good works in Christ's name. They passed along the testimony of Gaius assisting them in their journeys.

These travelers were obviously evangelists who were in the business of spreading the gospel among the Gentile nations. Because of the help of Gaius, the evangelists did not need to solicit support from the gentiles.

Because of Gaius support for their work, he became a helper of the spread of the Gospel even though he was not actively preaching the Gospel among the Gentiles. So because he sacrificed so the work could be accomplished, he was considered part of the work or a fellowshiper or a partner in the work and will be blessed by the Lord for his contribution.