Utopia, as you all may know, is the representation of an ideal world, alternative to the real world. This perfect world is a kind of criticism of the real world, is a improvement of it. This term was applied by Thomas More in his work “ De Optimo Republicae Statu deque Nova Insula Utopia”, where utopia was a term given to a fictional community which politic, economic and cultural organization is far from the human societies of his time. Although Thomas More was the creator of the term, the concept was older. It belongs to the “Res Publicae” from Plato, which Thomas More mentioned in his work.
Related to Gulliver's Travels, I want to stand out that the concept of Utopia in the sense of perfect organization of a society can be seen in book IV, which says that Houyhnhnms have a strict familiar plan, which dictates that if a family has two males or two females, one of the sons must be changed with other family to be always male and female.
Another thing I want to stand out is that Houyhnhnms work altogether, they are perfectly educated and it doesn't mind their identities. Here, we can make a contract with Gulliver that is well showed in the fourth book. While Gulliver doesn't have any sense of belonging to a nation, because he doesn't agree with his nation's situation; the Houyhnhnms are fused. I also want to mention that Gulliver doesn't feel like a native because he has spent almost his entire life at sea as an individual. I think that's why Gulliver doesn't want to leave this society, because it is the first time he feels like part of a society and he likes it.
I think that what Swift is trying to show with that attempt of feeling integrated into a society is a kind of mockery of that concept of Utopia and he showed his opinion by demonstrating that utopia is not possible in the sense that if there is a new individual that wants to enter this utopia it cannot. I think Swift also includes the concept of alienation in the sense that if Gulliver wants to enter the Houyhnhnms' society, he has to change his mind.