Gulliver's Travels is first and foremost a satire on the "adventure voyage" genre of fiction that was popular at the time, specifically Robinson Crusoe, which Swift is rumored to have despised for its European superiority and rejection of foreign culture as meritorious. However, several aspects of the novel are similar to those of science-fiction, especially the archetypal journey of a regular person (human from Earth) to far-off lands (other planets), meeting with strange creatures (aliens) and returning home with his story, which is met with disbelief.

Gulliver's journey to lands where the people are tiny or enormous is just as fantastic as meeting squid aliens on Jupiter. He even meets what might be termed as proper aliens, literal inhuman creatures called Houyhnhnms that are horse-like in appearance but entirely intelligent. Swift overturns the Euro-centric protagonist of common fiction by having Gulliver realize how inferior human culture is to that of the Houyhnhnms; unlike much of the sci-fi that followed, Gulliver is never able to affect or dominate other cultures with his own, finding that they consider him practically a barbarian.

Possibly the most sci-fi-like aspect of the novel is Gulliver's voyage to Laputa, the floating island, populated by brilliant scientists who are constrained by their minds and can barely function in society.

The flying or floating island is exactly circular, its diameter 7837 yards, or about four miles and a half, and consequently contains ten thousand acres.

[...]

Upon placing the magnet erect, with its attracting end towards the earth, the island descends; but when the repelling extremity points downwards, the island mounts directly upwards.
(Swift, Gulliver's Travels, gutenberg.org)

The imagery of a floating island has become a classic trope of science-fiction, and appears in works as diverse as the Cities in Flight series by James Blish, to 2009's Avatar by James Cameron. The image may well have appeared for the first time in fiction here, putting legends and mythology aside. It is even likely that Jules Verne or H.G. Wells used it as inspiration, being the commonly-assumed forefathers of sci-fi; certainly, they would have read it during their lifetimes. Overall, the use of different lands instead of different planets is the most important similiarity; Swift had no otherwordly context to place his extraordinary cultures, and so placed them on unexplored areas of the Earth, something which is less believable in the present day.