The way you need to cite "Gulliver's Travels" depends on whether you are using MLA, APA, Chicago or some other citation format system and the particular edition of the work you are reading.
One of the points of citations is so someone else can look at precisely the same page of a work that you are referencing. Since different editions have different numbers of words on a page, what is on page 100 of one edition may be on page 120 of another.
The information you need for citing this work is the author, title, publisher, and place and date of publication. If it is a critical edition, it may also have an editor. If the publisher has brought out several editions, it may have an edition number.This information is normally on the title page and the back of the title page of the book (this is one of the first pages inside the front cover and has the book's title on it in large letters).
Some citation styles use an in text citation (usually including author, year and page) combined with a works cited list and others use footnotes. Ask you teacher which style guide s/he prefers and then follow the appropriate guidelines in the resources below.