The Gospel of Truth was probably written in Greek between 140 and 180 by Valentinian Gnostics (or, as some posit, by Valentinus himself). It was known to Irenaeus of Lyons, who objected to its Gnostic content and declared it heresy. Irenaeus declares it one of the works of the disciples of "Valentinius", and the similarity of the work to others thought to be by Valentinus and his followers has made many scholars agree with Irenaeus on this point.
But the followers of Valentinus, putting away all fear, bring forward their own compositions and boast that they have more Gospels than really exist. Indeed their audacity has gone so far that they entitle their recent composition the Gospel of Truth, though it agrees in nothing with the Gospels of the apostles, and so no Gospel of theirs is free from blasphemy. For if what they produce is the Gospel of Truth, and is different from those which the apostles handed down to us, those who care to can learn how it can be show from the Scriptures themselves that [then] what is handed down from the apostles is not the Gospel of Truth.
After its Coptic translations and their burial at Nag Hammadi, the text had been lost until the Nag Hammadi discovery.
The text is written with strong poetic skill (notable even in translation), and includes a heavily cyclical presentation of themes. It is not a "gospel" in the sense of an account of the works of Jesus of Nazareth, but is better understood as a homily. The text is generally considered by scholars one of the best written texts in the whole Nag Hammadi collection, considering its worth highly as both a great literary work and a gnostic exegesis on several gospels, canonical and otherwise.
The work cites "Galatians, John, 1 John, Revelations, Matthew, 1 Corinthians, 2 Corinthians, Ephesians, Colossians, and Hebrews" from the NT. Of these it cites the Johannine Gospel the most often. It is also influenced by Thomas, for instance at one point (22.13-19) it cites John 3:8 alongside Thomas 28.
The text describes a theory of the rise of Error in personified (female) form. The ignorance and yearning to see the Father bred fear, which coalesced into a fog by which Error gained power.
It then describes Jesus as having been sent down by God to remove the ignorance. Jesus was a teacher confounding the other scribes and teachers, and asserted they were foolish since they tried to understand the world by analysing the law. But Error grew angry at this, and nailed Jesus to a tree. It also proceeds to describe how it is knowledge which grants salvation, which constitutes eternal rest, describing ignorance as a nightmare.
Having next described the parable of the good shepherd, in an esoteric manner, it then describes how feeding the hungry and giving rest to the weary is to be understood as feeding spiritual hunger, and resting the world weary.
This is followed by a parable about anointing, the meaning of which is obscure, but may be connected with the way in which a sealed amphora meant it was full, a metaphor for knowledge - having the final "seal" in the jigsaw and you understand, but without it, the scraps of understanding you have put together can still be easily undone:
Aside from a final description of achieving rest by gnosis, the remainder of the text concerns a treatise on the connection between the relationship between the Son and the Father, and the relationship of a name to its owner. The prime example of this is the phrase it uses that the name of the Father is the Son, which is to be understood in the esoteric manner that the Son is the name, rather than as meaning that Son was a name for the Father.
Layton printed eight fragments of Valentinian literature, each a quote which a Church Father claimed to take from the Valentinian corpus although none from the "Gospel of Truth". Layton further noted where the excerpts agree with one another.
"Fragment G", which Clement of Alexandria (Stromateis 6.52.3-4) related to "On Friends", asserts that there is shared matter between Gnostic Christian material, and material found in "publicly available books"; which is the result of "the law that is written in the [human] heart". Layton relates this to GTr 19.34 - when Jesus taught, "in their hearts appeared the living book of the living which is written in the Father's thought and intellect". Both rely on a shared concept of pre-existent yet obscured knowledge, which had emanated from the Father of the Gnostics.
"Fragment F" also comes from the Stromateis, 4.89.1-3. Directed to the Gnostics, it calls the congregation "children of eternal life" and hopes that they will "nullify the world without being yourselves nullified". Layton relates the former to GTr 43.22 at the end of the work, which emphasises that the Gnostics are the Father's children and will live eternally. Layton relates the latter to GTr 24.20, which proposes to "nullify the realm of appearance" and then explains this as the world which lacks the Father.