What did Gnostic groups actually believe, and were they suppressed?

It’s more complicated. “Gnosticism” is a broad label for several different movements, not one religion with a single agreed set of teachings. Some groups described the maker of the material world as a lesser or ignorant being, and emphasized special knowledge as a way toward spiritual understanding. But the claim that all religions before Christianity were deliberately created by an evil deity is too sweeping to describe them all.

There were real disputes between these groups and early Christian leaders, and some writings were rejected or stopped being copied. Still, texts associated with these traditions survived, including manuscripts found near Nag Hammadi in Egypt. “Suppressed” captures part of the story, but can make it sound like one unified movement was erased in a single campaign.

I’d be cautious with the conspiracy version. Early Christianity included competing ideas, and later writers argued over which teachings counted as acceptable. Some groups lost influence and their works became harder to find, but that doesn’t show that every pre-Christian religion was part of one plan to imprison humanity. Also, treating all Gnostic thought as simply hating the physical world leaves out the differences between groups.