Fact and fable are represented by two dueling accounts in the book. On the one hand, Gulliver accurately relates how his country and his fellow men act towards each other, how the government and society works, and how "normal" life is compared to the societies he encounters. On the other hand, Gulliver's journeys bring him to incredible places with astonishing people, things that are not possible but serve to showcase alternative lifestyles that Swift presents as possibly superior to his own society.
How low an opinion I had of human wisdom and integrity, when I was truly informed of the springs and motives of great enterprises and revolutions in the world, and of the contemptible accidents to which they owed their success.
(Swift, Gulliver's Travels, gutenberg.org)
Swift carefully sowed seeds of truth into the fantastic worlds that he created, causing readers to see themselves in a new light. By focusing on Gulliver's unceasing attempts to justify himself and his society, Swift is able to highlight the more ridiculous parts of society entirely under the guise of defending them. This all comes to a head in the final part of the book, when Gulliver is forced to admit the superiority of the Houyhnhnm society. By showcasing the worst attributes of human society in comparison with false societies that could not exist in the real world, Swift informs the reader of the fundamental silliness of many modern customs while also entertaining with stories of fantastic fictional worlds.