They “bore witness” to Gaius’s love before the church. Their report is presented as public confirmation of his faithful care (3 John 1:6).
In 3 John 1:5–8, the writer praises Gaius for steady, reliable help given to traveling believers, including ones he did not know personally. These visitors have publicly reported Gaius’s love to the local gathering. The writer urges Gaius to keep assisting them as they continue their trip, and to do so in a way that fits God’s character. This unit sits in the letter’s opening commendation of Gaius. Just before, the writer rejoices that Gaius is “walking in the truth” (3 John 1:3–4), and then shows what that looks like in practice: welcoming and resourcing traveling brothers.
The passage presents hospitality as a concrete expression of loyalty to the Christian message. The writer praises Gaius for “faithful” help given to traveling believers, including people he did not already know (explicit). Their public report to the community confirms that Gaius’s “love” was visible and recognized (explicit).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with 3 John 1:6, then read the surrounding section so the answer stays connected to the passage's flow.
6They have testified about your love before the assembly. You will do well to send them forward on their journey in a manner worthy of God,