How would I write a short essay on Gulliver's Travels?
The first thing to do is to become familiar with the book itself; Gulliver's Travels is available to read for free on eNotes, and there is a very good study guide and summary.
Next, pick a topic to write about. Most essays for this book focus on the satirical nature of the novel, and how it examines the real-life issues that Jonathan Swift experienced during his life. For example, a good topic for satire might focus on the examination of human culture as seen through foreign eyes; many of the cultures that Gulliver encounters are incredulous at his descriptions of British government. One excellent example of this is the Brobdingnagian horror at his description of gunpowder; Gulliver can't understand how a ruler would refuse to adopt a technology that gives him military power, while the Brobdingnagian king considers this a lesser accomplishment than the arts.
Another good topic would be to contrast the actual novel to its image in popular culture. The first section of the book is often adapted by itself and presented as a children's story. However, the book itself contains many concepts that children would not understand, as well as a great deal of scatological and obscene humor that Swift used to underscore human culture. Comparing the actual events of the book to the 2010 film, for example, would give an interesting perspective on how 18th century audiences perceived entertainment versus today.