When analysising Justin Martyr's writings in his three letters, we need to keep in mind his audience.
In his two letters of Apology to the Roman Emperor and Senate, he mentions the word "Gospel" once and the word "memoirs" three times. He explains what he means by "Gospel" and "apostle" in his First Apology:
http://www.earlychristianwritings.com/t ... ology.html
Accordingly, after He was crucified, even all His acquaintances forsook Him, having denied Him; and afterwards, when He had risen from the dead and appeared to them, and had taught them to read the prophecies in which all these things were foretold as coming to pass, and when they had seen Him ascending into heaven, and had believed, and had received power sent thence by Him upon them, and went to every race of men, they taught these things, and were called apostles.
For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body;" and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood;" and gave it to them alone. Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.
I've read arguments that Justin's scanty use of the word "Gospel" to refer to written materials meant that this was a new term for Christians, but in fact it seems to have been a new term for pagans, at least in context of the use of the word by Christians (as the word "gospel" had been around for a long time and used by pagans as well in its own context.)
In his
Dialogue with Trypho, Justin explains a little more about the authors of the memoirs:
http://www.earlychristianwritings.com/t ... rypho.html
For in the memoirs which I say were drawn up by His apostles and those who followed them, [it is recorded] that His sweat fell down like drops of blood while He was praying...
The "sweat like blood" is from Luke 22:44.