Time's Richard Lacayo starts off his summary on why this one is on the list with "no, it is not unreadable." He then describes the plot as "unsummarizable," which as far as I know isn't a word.
Readable books (which is the point of books) are not described as "not unreadable." Because that doesn't make any sense. For example, the following sentences are never said:
"No, the food at Arby's is not unedible, but the ingredients are unsummarizable."
"No, it was not unwatchable; I sat through all 110 minutes of 'United Passions'."
"No, the flight was not unsmooth; but our crash landing is unsummarizable as I have a head injury."
"No, he is not unattractive. The deformities and lesions on his face are, however, unsummarizable."
I don't really have much else to add about Gravity's Rainbow. After 112 days, it is finished.
Oh, by the way, the plot really is unsummarizable, hence I have not included a summary of it. I honestly have no idea what it was about.
This becomes the third book to take up last place in my rankings. The first being The Spy Who Came in from the Cold (as it was the first one I read), and Naked Lunch, which has held the title since December of 2010.
Next up is Vladimir Nabokov's second list book, Pale Fire.