Against Heresies/Adversus Haereses or On the Detection and Overthrow of the So-Called Gnosis by Irenaeus
https://www.earlychristianwritings.com/irenaeus.html
https://www.newadvent.org/fathers/0103.htm
http://gnosis.org/library/advh1.htm
Refutation of All Heresies by Hippolytus
https://www.newadvent.org/fathers/0501.htm
Epistle of Barnabas
https://www.earlychristianwritings.com/barnabas.html
The Shepherd of Hermas
https://www.earlychristianwritings.com/shepherd.html
https://www.newadvent.org/fathers/0201.htm
The Nag Hammadi Library
http://gnosis.org/naghamm/nhl.html
The Nag Hammadi Scriptures by Marvin Meyer et al.