In his excellent book "The Gospel as Manuscript," Chris Keith makes reference to a passage in The Epistle of Diognetus in the midst of a discussion over when the transition occurred from a "primarily" oral proclamation of the Jesus tradition to written texts.
"Often overlooked in this debate is the occurrence of εὐαγγέλια in the Epistle of Diognetus, where the term is paired with the law and the prophets and thus likely assumes a "book" meaning."The passage Keith is referring to is the following;
"Ίhen the reverence of the law is praised in song, and the grace of the prophets is recognized, and the faith of the gospels is established, and the tradition of the apostles is preserved, and the joy of the church exults." (Diogn. 11.6)
"εῖ̓τα φόβος νόμου ἄδεται καὶ προφητῶν χάρις γιςώσκεται καὶ εὐαγγελίων πίστις ἵδρυται καὶ ἀποστόλων παράδοσις φυλάσσεται καὶ ἐκκλεησίας <χαρὰ> σκιρτᾷ." (Diogn. 11.6; Holmes, 714-715)The date of composition for Diognetus is rather an open question, but somewhere around 150-225 is likely (according to Holmes, 689). Though proposals have been made that it was written by either Quadtratus (an early apologist, ca. 130 CE), or Polycarp (ca. 69-155 CE) (Holmes, 688-689). Either way the writing is generally accepted as being early and Diognetus appears to be referring to written material designated as "Gospels" (εὐαγγελίων) along side references to the Law and to the Prophets (obviously written material).
Added to this, I find it interesting that in the very next chapter the author of Diognetus is referring to its audience as "listening" to the contents of the epistle being read out (Diogn. 12.1). Thus, the immediate context is referring to the reading out of texts.
Chris Keith, "The Gospel as Manuscript: An Early History of the Jesus Tradition as Material Artifact" (New York: Oxford University Press, 2020).
Michael W. Holmes, "The Apostolic Fathers: Greek texts and English translations" (3rd ed. Grand Rapids: Baker Books, 2007).