This is to me one of the funniest parts of the story, since Gulliver takes it very personal and is at his angriest. Basically Gulliver told off his cousin. He was pretty upset because he claims that his cousin "sold him out" in terms of embellishing and making up stories. He was mad also because he wanted to change the world and its ruined behaviors in a period of seven months after his book got published. Yet, the way that his cousin twisted the story led people to believe that the world needed no fixing, that everything is OK and, as a result, his book became obsolete and unnecessary. Nobody received the "enlightenment" that Gulliver intended to give, and so for all the efforts that he went through he basically wasted his time. And its all his cousin's fault.
When you have a chance, read the actual text (linked below) because it is quite funny to even picture Gulliver expressing himself this way.