Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Gulliver is an active narrator: he often talks directly to us, the readers. When he does, he usually wants forgiveness for something or other. Take, for example, Gulliver's opening paragraph after describing the pitiful case of the immortal struldbrugs to us:
I thought this account of the struldbrugs might be some entertainment to the reader, because it seems to be a little out of the common way; at least I do not remember to have met the like in any book of travels that has come to my hands: and if I am deceived, my excuse must be, that it is necessary for travellers who describe the same country, very often to agree in dwelling on the same particulars, without deserving the censure of having borrowed or transcribed from those who wrote before them. (3.11.1)
Gulliver seems just a bit on the defensive side here, with his concerns about the "entertainment to the reader" – and in fact, this is a common trope throughout the novel. Gulliver often refers to his "maligners" (1.2.2), apologizes to the reader for taking so much time, or begs the reader not to judge his subjects too harshly due to this or that reason. However, this show of humility seems itself to be a joke. Gulliver regularly spends whole paragraphs on the technical details of (fictional) languages and (fake) customs. He takes his own sweet time in describing what he wants to, without too much apparent concern for "entertainment."
This leads us to our other adjective, "ironic." Gulliver tells us that he does not recall reading anything about the struldbrugs in "any book of travels that has come to [his] hands." Of course, he doesn't remember reading about the struldbrugs: they are an invention of Swift's own. Swift is sharing a joke with his readers about realism in writing. After all, just because something is carefully described or new to the reader doesn't make it true. Swift knows the struldbrugs aren't real and we know the struldbrugs aren't real, which makes Gulliver's protestations ironic and funny. By using this ironic tone, Gulliver's Travels parodies all those "book[s] of travels" that claim to be realistic and truthful because they include loads of detail.