An animated-cartoon style for this comic seems an odd choice when Dell did other such classic story one-shots with art by the likes of Jack Sparling. Lemuel Gulliver's short proportions are the first odd thing I noticed. But when eventually I zeroed in on the thicker ink lines on Gulliver's face throughout, I realized he'd been redrawn, and then the nickel dropped.
Magoo's cartoonishly bulbous nose has been replaced with a more nearly realistic one and the bags under his eyes gotten rid of. But the one time an ear is seen under Gulliver's long hair, it's Magoo's—the top is folded down—and his jowls and chin are still obvious. The cover Gulliver's face doesn't much resemble the interior Gulliver's—his chin is not at all as pronounced; the cover was no doubt drawn after the licensing fell though.
The 1962 TV special "Mister Magoo's Christmas Carol," in which he acted the role of Ebenezer Scrooge, was successful enough that after the special's 1963 repeat "The Famous Adventures of Mister Magoo" came out as an ongoing primetime series in the 1964-65 season. Magoo (probably wearing contact lenses as an actor, as his nearsightedness was not in evidence) played such characters as D'Artagnan, Gunga Din, and Victor Frankenstein. Dell's Gulliver's Travels was published after the last of the original episodes was shown in 1965, but it had to have been begun as a tie-in to the show. Magoo never played Gulliver on the series; the comic was an original story in that sense.
The fact that the artist of Gulliver's Travels drew Dell's Mister Magoo numbers 4 and 5 two years earlier is obscured by that facial retouching but is, I think, evident in the supporting characters.
UPDATE: Mark Evanier suggests longtime Mort Walker assistant Bob Gustavson as a possibility, and I think that's exactly who drew Mister Magoo 4 and 5 and Gulliver's Travels.