After finally reading The Gnostic Religion: The Message of the Alien God & the Beginnings of Christianity, one can gain some insights into how Philip K. Dick combined his philosophic interests in idealism and metaphysics with Gnostic and Christian themes that predominate in his later works. While I am not entirely certain of this, one cannot help but feel Dick read the work of Jonas on Gnosticism. The timing works, mid-1960s and all. And Jonas wrote a nice overview of Gnosticism but I think more contemporary scholarship has benefited from more complete access to the Nag Hammadi finds and other sources.
One also thinks some are ready to jettison the concept of a singular "Gnosticism' since it consisted of so many different groups and sects which did not necessarily share much. The church fathers who dismissed them as heretics or Plotinus the neoplatonist may have grouped followers of Valentinus and others together as heretics, but that does not necessarily unite the followers of Valentinus and Mani with the Mandeans or followers of the Corpus Hermeticum. Hans Jonas himself admits this several times in various chapters on Valentinians, Macrosians, Manichaeans, Mandaeans, etc.
However, they all shared some similar core concepts that can ultimately be traced to what he sees as three important influences: Iranian dualism, Babylonian astrology, and Jewish monotheism, within a Hellenistic framework. One cannot discount the context of the Roman Empire and the shift from the polis to imperial governance. Despite this Hellenistic framework, there was nonetheless very different cosmogonies and biblical exegesis in the various schools of 'Gnosticism.' Manichaeanism and Mandaeans seem very distinct from each other, with various sects being more overtly anti-Jewish, equating the demiurge with the Old Testament God. The Babylonian astrology sources enter through Hellenistic astrology in Egypt and the rest of the Mediterranean world, with the various planets/celestial spheres and stars becoming associated with various names of the Old Testament God or aeons.
It's a sometimes bewildering hodgepodge of various schools of thought in religion, mysticism, and but as Jonas says, some Gnostics would have been horrifying to Christians for their vastly different interpretations of Scriptures (sometimes equating the Edenic serpent as a positive force, for instance) as well as rejecting works or or the moral "laws" of the false material world. Furthermore, by rejecting the celestial spheres and the cosmos as the imprisoning world of a lesser being, Gnostics would have been heretical or rebellious to the earlier Greek notion of the cosmos and Neoplatonists like Plotinus or the Stoics. Either the "Gnostics' became libertines or ascetics, rejecting the physical world as a creation of the Darkness/Demiurge/lesser god.
In the epilogue, Jonas tries to connect Gnosticism and existentialism, but I don't see much of any connection besides what he calls a shared "anthropological anticosmism." But the comparison is revealing nonetheless, since some of the parallels in the world ancient world he draws may have influenced science fiction writers like Dick. For example, the rise of the Roman Empire and the the post-WWII US hegemony (or US and USSR "Empires" as Eastern and Western halves of the same imperial power) may have appealed to Dick's sense of the post-WWII global "empire" of the US/USSR, and the need to escape the totalitarian/all-encompassing "fake" world of the 20th century with its false reality that binds us to this realm. I cannot say for certain that Dick read Jonas, but it is certainly possible his own theophany and interests in Zoroastrianism and Gnosticism were informed by him. The influence of astrology and ancient astronomy on Gnostic thought could have also appealed to writers of science fiction, which may contribute to the omnipresence of Gnostic themes in various forms of SF from Dick's novels to anime series like Ergo Proxy.