[Teresa Edgerton]

Although in Lud-in-the-Mist you do have the direct intervention of fairies and ghosts in human affairs (for all the efforts of the citizens of Lud to obscure the matter with their legal fictions about smuggled silk).

Here are definitions from three dictionaries:


(Merriam-Webster's Online Dictionary)

fan·ta·sy
Variant(s):
also phan·ta·sy \ˈfan-tə-sē, -zē\
Function:
noun
Inflected Form(s):
plural fan·ta·sies
Etymology:
Middle English fantasie — more at fancy
Date:
14th century
1obsolete : hallucination
2: fancy; especially : the free play of creative imagination
3: a creation of the imaginative faculty whether expressed or merely conceived: as a: a fanciful design or invention b: a chimerical or fantastic notion c: fantasia 1 d: imaginative fiction featuring especially strange settings and grotesque characters —called also fantasy fiction
4: caprice
5: the power or process of creating especially unrealistic or improbable mental images in response to psychological need <an object of fantasy>; also : a mental image or a series of mental images (as a daydream) so created <sexual fantasies of adolescence>


(Oxford University Press)

fantasy, a general term for any kind of fictional work that is not primarily devoted to realistic representation of the known world. The category includes several literary genres (e.g. dream vision, fable, fairy tale, romance, science fiction) describing imagined worlds in which magical powers and other impossibilities are accepted. Recent theorists of fantasy have attempted to distinguish more precisely between the self‐contained magical realms of the marvellous, the psychologically explicable delusions of the uncanny, and the inexplicable meeting of both in the fantastic.


(American Heritage Dictionary)

an·ta·sy (fān'tə-sē, -zē) Pronunciation Key
n. pl. fan·ta·sies
The creative imagination; unrestrained fancy. See Synonyms at imagination.
Something, such as an invention, that is a creation of the fancy.
A capricious or fantastic idea; a conceit.
Fiction characterized by highly fanciful or supernatural elements.
An example of such fiction.
An imagined event or sequence of mental images, such as a daydream, usually fulfilling a wish or psychological need.
An unrealistic or improbable supposition.
Music See fantasia.
A coin issued especially by a questionable authority and not intended for use as currency.
Obsolete A hallucination.


None of these say that magic or the supernatural have to be present.

Going by the Merriam-Webster definition, Gormenghast definitely qualifies.