Funny thing, I also recalled Hexen and Doom recently as close associations to Gems of Provenance: non-linear, tangled and sometimes mazelike interconnected levels, lots of secrets (some of which are well-hidden), more or less thin story (almost absense of long readables and dialogs, the story is almost non-existent in those games and very slight in Gems), general preference of ambiance and strong environmental storytelling... In general I most like first and last levels (and also Heart and So...