This sort of literary claptrap tends not to mean very much in my opinion. But I'll have a go at interpreting it. Let's start at the end and work backward.
"Stylized contestation." The author says that Gothic Fiction contains stylized conflict. This conflict, in my opinion, is usually between forces of good and those of evil. The word stylized indicates that the conflict is not portrayed realistically. But then, having said that, the author uses the phrase "fancy-dress disguise", which indicates that the stylized contestation is not really the point of Gothic Fiction. Regrettably, he does not favour us with his opinion as to what Gothic fiction is really about, if not the struggle between good and evil.
Anyway, that context of fancy-dress disguise of stylized contestation is the milieu in which the action occurs. And what is the action? It is Gothic Fiction's ability to express the voice of the "other" The "other" refers to people or beings that are unlike the author and the reader. The reader presumably sees himself as good, and the "other" may be evil. Or the "other" may be a former human being who has somehow been spoiled or corrupted, like Count Dracula for example. The "other" might also be an artificial pseudo-human like Dr Frankenstein's monster. The "other" is everything that is alien to the reader.
So to sum up, without in any way explaining how, the author contends that Gothic Fiction is able express the voice of that which is alien to us, within the context of its stylized conflict, which in turn is merely an affectation and not the essence of Gothic Fiction.
I hope that helped.