Somewhat depends on exactly what about Gormenghast you want something similar to, but I have quite a few! Mordew by Alex Pheby Piranesi by Susanna Clarke Jonathan Strange & Mr Norrell - Susannah Clarke. Almost anything by China Miéville. I'd recommend Perdido Street Station Book of the New Sun by Gene Wolfe Gloriana by Michael Moorcock Then there are some other WeirdLit recommendations that are slightly further away but share some aspects you may appreciate, like Vandermeer's Ambergris trilogy