Both Geoffrey Chaucer's The Canterbury Tales, and Jonathan Swift's Gulliver's Travels, are forms of satire. The Canterbury Tales is a form of mellow satire, whereas Gulliver's Travels is more harsh.
...the literary art of ridiculing a folly or vice in order to expose or correct it. The object of satire is usually some human frailty; people, institutions, ideas, and things are all fair game for satirists. Satire evokes attitudes of amusement, contempt, scorn, or indignation toward its faulty subject in the hope of somehow improving it.
Geoffrey Chaucer's satire is more mellow because he presents the foibles and hypocrisies of those involved with the medieval Church and the common folk traveling on a religious pilgrimage in a comic way. (In fact, most of Chaucer's criticisms are found "between the lines.) The Church may not have been pleased with his writing, but Chaucer, a student of human nature, observed the human condition and allowed his audience to draw conclusions seemingly without an intent to stir public opinion into a frenzy.
Jonathan Swift's satire is much more harsh, dealing more with politics; and while people at first thought it was simply an adventure story, when the true intent of his writing was finally realized, people were particularly angry. eNotes.com's "Critical Overview" states:
Writer William Makepeace Thackeray said the message of the book was 'horrible, shameful, blasphemous… filthy in word, filthy in thought' and 'obscene,' and certainly proof that Jonathan Swift was 'about the most wretched being in God's world.'
Whereas Chaucer wanted people to be aware of the hypocrisy of the Church (especially servants of the Church as opposed to the lay person), Swift is much harsher, finding fault with the government to the point that people turned on him and blamed his personal life for the criticisms he raised, rather than looking to the political system and evaluating whether it should be altered in some way.
Those with unwavering respect for Swift's work suggest, on the other hand, that some of his readers confused the voice of Gulliver to the heart of Swift, thereby believing that Swift was Gulliver. The voice of a character can say very different things, or say them in a much different way, than the author.
It seems that Swift, a religious man, was deeply troubled with what he saw of human nature, and much of this was tied to politics as if was practiced in England, and even as it affected the Irish. (This also comes to light in his harsh satire, "A Modest Proposal." Swift saw the realistic ramifications personally while living in exile.)
Swift had hoped to "alert people to the ugliness of human behavior."