One cannot fully understand or appreciate Lemuel Gulliver as a character without first understanding that Jonathan Swift was a satirist and this novel is written as a satire of European culture/society, especially England. In the context of being a satirical novel, Gulliver as a character becomes a vehicle for Swift's delivery of satire. I would disagree with the first commenter in that Gulliver is meant to represent the reader of the novel because doing so would entirely undercut the satire and ignores the historical context of Swift's writing. Swift was not interested in writing a silly adventure because in 1703 tensions between England and his native Ireland were very turbulent and, at times, quite violent. Swift, having become quite jaded and cynical himself, set out to draw attention to the flaws of the English socio-political climate and economic policies through satire.
So when Gulliver encounters the Lilliputans (who can see with great exactness, but not at a distance), and then the Brobdingnagians (who can see at great distance but without exactness), both groups ridicule the European ways and methods he explains to them. And because Gulliver becomes a sympathetic character, perhaps because he is the only character relatable to the reader, the reader is expected to share in Gulliver's shame and confusion when his beloved England is ridiculed. In both of these places it also becomes obvious that he cannot safely remain a resident. In Lilliput he is feared for his size and potential for destruction (they did, after all, weaponize him to attack Blefuscu) to the point that they litigate him out of the equation to avoid the costs of maintaining him. In Brobdingnag he is so insignificant that he is at risk of being forgotten and neglected, which is the opposite of the problem he had in Lilliput. His money is invisible dust to the king of Brobdingnag while in Lilliput his money is a uselessly enormous burden to carry.
All the while, Gulliver himself remains surprisingly emotionally objective, allowing his grasp on his Christian virtue to determine his actions and reactions. In Lilliput he does not destroy the fleet of Blefuscu because he finds a more rational solution that is completely unfathomable to the Lilliputans and ultimately earns him charges of treason. In Brobdingnag his virtue allows him to maintain his dignity is the face of humiliation and ridicule at the hands of the king.
Part 3 is Swift's satiric ridicule of science and education and Part 4 satirizes philosophy and religion.
The absurdity of what happens in the novel can deceive the reader to think that this is just a wacky adventure (it certainly tricked Jack Black into making that awful, awful movie), but absurdity is what makes the satire all the more powerful. Gulliver is the static constant throughout the entire novel, hardly changing his virtue or disposition in spite of everything he sees and experiences. Instead, he at times even closely resembles a straw man who is there simply to ask questions about the strange things he sees.