Gulliver claims that absolutely everything he has written is absolutely true.
In fact, he thinks it's a disgrace that so many travelers embroider or exaggerate their published accounts of their trips around the world.
Gulliver's motto is: Nec si miserum Fortuna Sinonem/Finxit, vanum etiam, menacemque improba finget () – "Though Fortune has made Sinon wretched, she has not made him untrue and a liar." (citation: Robert Greenberg, Editor, Gulliver's Travels: An Annotated Text With Critical Essays . New York: Norton, 1961, 256). In other words, though Gulliver is bummed about having left Houyhnhnm Land, he still refuses to lie about any of his experiences.
The purpose of writing his memoirs is not to gain fame, but to share the superior example of the Houyhnhnms with the world.
Gulliver has been warned that he must first relate his experiences to an English secretary of state in order to give England the opportunity of invading the lands he has visited.
It wouldn't be profitable to try: the Lilliputians are too small to be worth it, the Brobdingnagians, too large and dangerous, and the Laputians, literally out of reach.
While the Houyhnhnms are totally inexperienced with war, still, the English shouldn't invade them.
The Houyhnhnms are smart, strong, and love their country – they would figure out how to defend it quickly enough.
In fact, Gulliver wishes that the Houyhnhnms would come over and teach all of their virtues to the European Yahoos.
A further reason why Gulliver doesn't want the Europeans to conquer the lands he has seen is because they don't seem to want to be conquered.
Taking their lands against their will is cruel.
So now, Gulliver is nearing the end of his tale.
Gulliver is sitting in his garden thinking; he is instructing his family as best he can; he is applying the lessons of Houyhnhnm Land; he is looking at his face in the mirror to get used to the features of Yahoos; and he is mourning the treatment of Houyhnhnms in England.
Just this last week (after five years home), Gulliver is able to let his wife sit at dinner with him – at the far end of the table.
What he really hates is not the bad qualities that Yahoos can't seem to escape. It's the pride they feel in themselves even though they are so disgusting, diseased, and detestable.
The Houyhnhnms, who possess good natures, are not proud, because they are born good, and cannot help but be good. They don't need to congratulate themselves.
The only way that Gulliver will ever be able to sit in the company of an English Yahoo again is if they avoid at least this one sin: the sin of pride.