Gnostic texts are mainly composed of allegories that act as instructions for meditation, similar to how the Bhagavad Gita can be read as a manual for yoga, and the insights gained through meditation. In that sense, any time a Gnostic writes a book on their own experiences, these can contain important insights on par with the original texts. This is especially true for eclectic Gnostics and Gnostic alchemists, but it tends to matter a lot less to, say, Valentinians or Cathars, since they have their own canon that covers pretty much all of their needs. There's also, as aforementioned, a decent New Age presence in the modern Gnostic movement, with figures like Alice Bailey and Samuel Aun Weor, who both wrote their own Gnostic texts. I do not hold a very high opinion of this approach to Gnosticism, but I also don't see it as my place to say they aren't "really Gnostic." It feels too close to a No True Scotsman to me.