It took me two attempts to get through Gnomon. I think the main theme is "privacy": What is ultimate privacy? What is it's ultimate lack? What might someone, or indeed a society, give it up for? Is that trade worth it? How does a person, who values it, live in society that doesn't? How does that society see that person? Etc...
I think the much more interesting question gnomon asks is: What if multiple narrators/narratives wasn't? In the same vein that Gone away world's was: what if narrator wasn't? -trying to be spoiler free.
With that said, I think the answers to those questions are disappointing in their respective books. In a rare display of focus for Harkaway, he ignores the fascinating can of worms he just opened in order to concentrate on the main theme.
[These questions are given weight because they are at the heart of the the twists. So interesting question, given narrative weight and subsequently ignored]
Harkaway is best when he is taking you down the garden path of batshit characters, doing batshit things, at batshit speeds.
I usually give out either Goneaway or Angelmaker (depending on how they like the idea of steampunk bees) with the instruction of "just go along for the ride" and they are so much fun like that.
Gnomon is harder to recommend because that ride is so disjointed and, on the surface, utterly irrelevant to other parts of itself that it's no fun at all. Only towards the end do you learn it's been the same ride all along, but by then it's too late.