I'm not sure what to suggest re Gormenghast. Dickens is definitely in there (especially the Prunesquallor episodes) and perhaps Peacock as well. I've only read a bit of Peacock, but the characterisation makes me think of him. But it also reminds me of lighter books like Cold Comfort Farm in its grotesqueness (and also in some of the landscape and buildings, which are both pretty absurd). And of course there are a lot of Peake's own experiences in there: the Forbidden City in China, his time on Sark, and (in Titus Alone especially) his time as a war artist. There might even be a bit of Bruegel or Blake in the mix. I'm sure I'll think of some more options (perhaps even Lewis Carroll, who Peake illustrated?).
Incidentally, I suspect everyone has a least favourite aspect of Gormenghast. I find the Keda interludes pretty boring, and the teachers always left me a bit cold.
One of the problems in pinning Gormenghast down is the tone: it's not quite a load of jolly eccentrics being splendid, but nor is it miserable grimdark. Interestingly, it seems to anticipate both goth and steampunk in popular culture, but they're probably influenced by it, to an extent - the Cure did a song about Fuschia.