26 Demetrius is apparently someone Gaius would have heard about, but whose character was not known to him. Thus the author is writing to Gaius to attest to Demetrius’ good character. It appears that Demetrius is coming to Gaius’ church and needs hospitality and assistance, so the author is writing to commend him to Gaius and vouch for him. It is difficult to know more about Demetrius with any certainty, but the author is willing to give him a powerful personal endorsement (We testify to him too). Demetrius may well have been the leader of a delegation of traveling missionaries, and may even have been the bearer of this letter to Gaius. The writing of letters of introduction to be carried along by representatives or missionaries in NT times is also attested in Paul’s writings (1 Cor 16:3).
27 The words “to him” are not in the Greek text, but are implied.

In other versions

Demetrius has received a good testimony from everyone, and from the truth itself. We also add our testimony, and you know that our testimony is true.

Demetrius is well spoken of by everyone—and even by the truth itself. We also speak well of him, and you know that our testimony is true.

Everyone speaks well of Demetrius—even the truth itself. And we also speak well of him, and you know that our testimony is true.

Everyone speaks highly of Demetrius, as does the truth itself. We ourselves can say the same for him, and you know we speak the truth.

Demetrius has been testified to by all, even by the truth itself. And we also testify to him, and you know that our testimony is true.

Demetrius hath good report of all men, and of the truth itself: yea, and we also bear record; and ye know that our record is true.

Public Domain