Gulliver tells us a lot, especially about himself. Whether we believe him or not is definitely in question, since much of the narration is ironic at the expense of Gulliver himself. Still, we can't take away the fact that Gulliver's direct characterization influences much of our sense of character in this book. For example, he writes: "Nothing but an extreme love of truth could have hindered me from concealing this part of my story" (2.7.1). He tells us that he has "an extreme love of truth" (and also that he prefers lying under some circumstances). Precisely the extremity of his "love of truth" makes us think that he's protesting too much – maybe he doesn't really love truth at all! But what we can get from Gulliver's direct characterization here is that the appearance of truth is important to him, whether he actually tells it or not.
Location is all-important in Gulliver's Travels. If you're living on Brobdingnag, chances are you're 60 feet tall and morally superior to the tiny Lilliputians. If you're living in Laputa, chances are you're feeling a bit of a breeze in your hair because you are flying, baby! Location is one of the key tools of characterization specific to Gulliver's Travels. Check out our "Characters" for more on what each of these locations actually mean.
Again, much like location, this is the other primary determining characterization tool of Gulliver's Travels: if you're 60 feet tall, you're probably wiser than if you're 6 inches tall. If you look like a horse, you're infinitely better than if you look like a shaggy beast thing. Because everyone in this novel symbolizes some idea or vice, outward appearances mean everything – these aren't characters, they're walking symbols, and appearance goes along with that.