The opening of the novel presents a picture of Gulliver as a somewhat arrogant man with incredibly high expectations.  His outrage at the changes made by the publisher without his approval is only amplified by his disappointment that his description of the Houyhnhms has not fundamentally changed society to something better.

This arrogance is only encouraged when he is among the Lilliputians as he is a giant and has incredible power for good or evil as he is among them.  His confidence in his own power is somewhat mitigated by his frustration and disappointment in the society of the Lilliputians but his arrogance is still rather apparent.

In Book II, suddenly he is suddenly helpless and small amidst the Brobdingnagians, a circumstance that helps to remind him of his frailty.  Though he is still cynical and critical about the society and government of the Brobdingnagians, serving to satirize specific elements of both the Tory and Whig conflict and other English affairs at the time, he becomes more aware of his own condition and this tempers his arrogance severely.

How would Gulliver be characterized in Parts I and II of Gulliver's Travels?

Principal characteristics that stand out in the character of Gulliver are the way that he seems to lack any canny or astute ability to get himself out of trouble. It is this trait above all that separates him from other heroes in texts such as Homer's The Odyssey, where Odysseus shows himself to be intelligent and quick-thinking. Gulliver, by contrast, is often a passive character who lets other characters do what they want with him without trying to resist or think his way out of the various situations he finds himself in. This is why he is a captive at various stages in his ventures. Remember that he is never released through his own plans or ability to think his way out of his troubles, and is dependent on circumstantial factors for his release. Although he is hardworking in pursuing his liberty when he sees an opportunity to escape, such as when he repairs the boat that allows him to leave Blefuscu, he is never actively engaged in coming up with strategems for escape himself.

Gulliver, as his name suggests, is also shown to be rather naive and gullible. A great example of this is the way that he is completely clueless about the way in which he is exploited by the Lilliputians. He may be very skilled in such areas as navigation and seafaring, but the text makes it clear that he lacks the ability to reflect on his own character and culture. He shows to lack any ability to philosophically reflect on the differences between humans and the people that he encounters.