Gothic literature is characterized by extremes, appealing both to the readers' emotions and their imaginations. These exaggerations are used to excite many emotions, including fear, terror, desire, distrust, and love. This style of writing is also characterized by transgression, that is, going beyond the accepted moral and social boundaries of society. These acts which are seen as outrageous, then, are designed to evoke the exaggerated emotions such as fear and distrust.
Gothic tales are often set in detached places, which contributes to the eeriness and strangeness of the plot. The atmosphere is often menacing in some way, by physical elements or characters or both. Gothic stories also often contain helpless victims (usually women) and predators (usually men or some kind of supernatural creature).
Angela Carter is a Postmodern Gothic writer who routinely employs such Gothic elements into her novels in order to reveal and address contemporary problems. That is true of The Magic Toyshop. (Even the title indicates either some kind of whimsical fairy tale or some grotesque parody of toys somehow coming to life and causing havoc and destruction.) In this novel she writes primarily to expose the confusion about gender, identity, and free will.
It is no surprise that one of the primary symbolic elements in the story is a puppet. Puppets are passive and possess no will or autonomy; they are at the mercy of the puppet-master. Both fifteen-year-old Melanie and her aunt Margaret are puppets being manipulated by the almost omnipotent Uncle Philip, the antagonist in this novel.
Melanie sees her uncle this way:
She saw her uncle only at mealtimes, but his presence, brooding and oppressive, filled the house. She walked warily as if his colourless eyes were judging and assessing her all the time. She trembled involuntarily when she saw him.... She sensed his irrational violence int he air about him.
Philip is even more oppressive with his wife, Margaret, even making her wear a necklace, which is more like a collar, on Sundays which nearly chokes her.
Later he forces Melanie to become Leda, the woman who is raped by the swan, instead of using a puppet. His object in doing this, of course, is to give vent to his own sexual fantasies of raping the young girl.
Obviously Philip is cruel to more than just the females in his life, as evidenced by his treatment of Finn. Finn does finally break free, at least to some extent, with Melanie's help, and is able to cut the almost-literal puppet strings which Philip was using to control him.
So now the question is whether the Gothic elements of the story are the moral lesson or if those Gothic elements are simply a vehicle for truth-telling. These two things are not mutually exclusive, and there is room for for both of these things to be true, it seems to me. It does, however, appear that one is stronger than the other in this particular novel.
The primary Gothic image in this novel is the puppet--not figurative puppets, but fantastical, life-sized puppets. In this setting, they are intimidating and grotesque and representative of the powerlessness puppets have in the hands of a puppeteer. The moral lesson (freedom of identity vs. manipulative control) is directly delivered through this Gothic element, making the Gothic aspect of the novel more than just the vessel by which truth is revealed. Take out this Gothic element, and the moral lesson would be much more difficult to identify. Not impossible, just more difficult.