Leaving aside the question of whether this book is a 'gothic' novel or not, it is a dark fantasy. It shares with its more conventionally gothic brethren a tale of dark deeds in an exotic setting, where an alien and exciting religion is practised.
In the standard Gothic tale, allusions to Roman Catholicism, thought of by respectable Englishmen as a dark, oppressive, and half-pagan faith, were part of the conventional apparatus. Beckford chose instead to imagine the world of Islam, an even more exotic milieu that added some flashes of bright colours to the dark and sorcerous background of his book. His choice of an even more exotic setting allowed him greater freedom in portraying characters who defied social convention and fell into exotic habits of mind.
My understanding is that it is a matter of some debate to what extent the English text of -Vathek- is a translation from the French, or an original English composition. I do not have the French text in front of me, but it has been represented to me that Beckford's "original" French is rather like the French of Oscar Wilde's -Salome-, and needed extensive editing to be acceptable to a French readership.
At any rate, -Vathek- is a prime example of early dark fantasy. The description, of course, will be richer than you are used to, but Beckford's prose actually seems to move quite quickly. Fans of H. P. Lovecraft or Clark Ashton Smith will find that it is quite easy to slip into. And the tale is indeed a vivid one, right up to the exceptional ending when Vathek and Carathis are damned to the halls of Eblis, their hearts seared with unquenchable fire.
This is a good edition of the story, and the notes and maps are helpful.