"Beloved, thou doest faithfully whatsoever thou doest to the brethren, and to strangers;
Which have borne witness of thy charity [love] before the church:"
3 John 1:5
The Bible says that faith expresses itself through love.
Gaius was faithful, he believed God and was full of faith.
His faith was full of love and his love was full of action.
The Church gave witness to his love and John commended him for his faithfulness.