I have to say didn't find it all that surprising that another author would adopt that stance. I think Gaiman is right, but I also think he's wrong.
If you decide to blog about your books, then you have an obligation to keep your fans up to date. Not a legal one, but a moral one. Also, what is the blog for? If you find you no longer want to keep your fans abreast of developments, either say so or discontinue the blog. Failing to do either is both annoying and dishonest.
"You're complaining about George doing other things than writing the books you want to read as if your buying the first book in the series was a contract with him: that you would pay over your ten dollars, and George for his part would spend every waking hour until the series was done, writing the rest of the books for you.
No such contract existed. You were paying your ten dollars for the book you were reading, and I assume that you enjoyed it because you want to know what happens next."
Of course a contract exists between author and reader. Again not a legal one, but a moral one. If you start writing a series and announce that it will feature x volumes, you have created for yourself a moral obligation towards your readers. Presumably you hope that people will like your books and if they do they will engage with the story and spend time not just reading, but thinking, speculating and dreaming, all of which strengthens the reader's emotional investment in the story. Many will do far more than that, as the number of extensive wikis dedicated to series and books prove.
You say you will deliver something and on the strength of that promise, lots of people make an investment of money, time, thought and emotion. If you don't consider that to be a moral contract, then you should never, ever begin writing a series. You should also state so clearly beforehand, so that I can avoid buying your books until your series is well and truly done.
I do agree, obviously, that no-one should or can be expected to spend every waking hour writing. Writer's blocks also occur and can last for ages. That's where your blog comes in though - if you have one, and Martin does, then you bloody well ought to keep your readers abreast of things at semi-regular intervals. Even if "things" amount to not a word having been written lately.