Most airlines allow GPS during cruise (e.g. KLM explicitly states the fact on its web site), but sometimes the cabin crew aren't aware of this.
OTOH some airline's documentation is quite ambiguous (I've just checked the advice my lot issue and it's not at all clear).
What I would say to the OP is if he does decide to try using it in flight please don't get into a "I know my rights" argument if asked to turn it off by one of the crew...