Um yeah, Kay's book does do that. It's basically Erik's biography, explaining just how it was he came to be skulking around the Paris opera house scaring people. It's interesting stuff, but it does screw up his mysterious aura.
As for the gothic thing, I forget how it's defined, but I'd always thought Leroux's novel was very gothic. Supernatural elements aren't an essential element of the genre, and there's all this Radcliffe/Walopole type stuff going on: imprisoned women, enclosed settings, a lot of the action taking place underground, odd murders, stuff like that. The set up and the scenario is very gothic, IIRC."
