Gulliver witnessed greed in lots of different ways, until in the end he becomes convinced it is one of humanity's principle vices.
The story shows us greed when the giant (Brobdingnagians) farmer chooses to display Gulliver for money, exploiting a creature who isn't really so different from him for profit.
The sailors on his ship (pirates he accidentally replaced his dead sailors with) mutiny out of greed.
Perhaps the best example of greed occurs when Gulliver comes to the land of the horses and discovers the Houyhnhnm Land Yahoos. The Yahoos (people) are typically deeply selfish and don't know how to share. They get into arguments over food—or occasionally for no reason at all. They like shiny things (gold and silver), and covet them above all else, stealing and hiding them when they cannot be obtained other ways. Gulliver recognizes the parallels between the Yahoos and the people of Europe (citing lawyers, who make money by defending people not worthy of defense and saying things they don't believe, and doctors, who encourage people to believe they are weak in order to get more money and try cures they aren't certain of also for money, and government officials, who profit off the people without doing anything for them).