Gnosticism most clearly was considered and talked about historically as part of Christianity by scholars.
originating in Jewish-Christian milieux in the first and second century AD
That doesn't make them Christian as Christian is commonly understood or as you have defined the term. Moreover, Mandaeanism and Manichaeism are arguably closer to the Persian religious tradition. And many Gnostic sects clearly owe a fair bit to Neoplatonism. So I'm not sure it's quite as straightforward as that.
The challenge with this all however is that not all sects of Christianity that were called Gnostic believed in things like the Demiurge as creator of the material world.
Perhaps not. But a fair few, perhaps even the majority did.
Did Valentinus believe that? I'm not sure of that.
I believe Valentinus did have a place for the Demiurge in his system.
Do you have direct, mystical experiences, or are your beliefs arrived at through rational and intellectual processes?
Both.