Gnostics basically believe that the "God" of the Bible basically IS Satan, and the entirety of the Bible is mostly his PR campaign to fool you into worshiping him while he shackles you to the material plane and keeps you from ascending to your true potential.
The "real" God in that scenario is the true creator of the universe who is a purely spiritual being who is responsible for the creation of all of our souls, and thus our free will and potential for transcendence, but the meat our souls are trapped in and the material world we live in are all the work of the evil god who is opposed to the true God.
That thinking has been at the heart of a number of heretical movements through history - the Albigensian Crusade in 13th century France was more or less rooting out this exact philosophy, and it's where the phrase "Kill them all, God will sort them out" originally comes from (though the original version was more along the lines of "Kill them all, God shall know His own").