The fact is nobody actually knows (but Kubo himself). Personally, I would just put his status as "unknown" (even though as a fan I do want him to be alive) - we don't know whether he died from the last attack from Nnoitra (although Ichigo stopped him before he could deliver another, presumed, final blow) or he's been magically rescued. I'm going to simplify for you some details I've seen/read about:
1. People have said in an interview Kubo says he's dead, though apparently this interview was fabricated. All I know is something as important as that (as Grimmjow gathered such a big fan base) would have been reported by well established sources - even if a copy of the interview wasn't avaliable.
2. Also, I've seen people on some forums where posters use page 17 of chapter 379 where Ichigo says Yammy's "comrades have died" (he mentions Grimmjow, Nnoitra and Ulquiorra beforehand) as source however other versions I've read have replaced "died" with "dying" (while, not good is not dead). I don't now if someone who can read Japanese can give us a good translation from an official copy of Shounen Jump to verify which word was used, maybe we can get a clearer answer.
3. In th 2nd character book, Grimmjow's status is "dying" (as mentioned dying =/= dead)
4. Again, the hearsay is the 3rd characters SOULs book (unmasked) will confirm Grimmjow's status. Again all hearsay/rumours things flying about on forums. Personally, all we can do is wait and see if this is true - this is one massive fact/plothole Kubo has created and I don't think fans have the patience to let him get away with it...
So I know you probably wanted a simple "yes, he's dead" or "no, he's alive" but the fact of the matter is whatever people tell you will be hearsay/their own theories. While some people give compelling answers, unless they are Tite Kubo, then the fact is they cannot give you an accurate answer. Best is to (patiently) wait for Kubo/official sources to report on the matter