I hope the answer is 'no' ! I use my Garmin on most flights and it is very rare to find explicit advice in the inflight mag or elsewhere on whether or not it is prohibited. They all go on about mobile phones and now also anything with earphones, but rarely mention GPS.
On a recent BA flight the announcement was to 'check in the Highlife Mag for more details' ...but there were none.
So until I hear otherwise I will still be the pillock who taps on the cockpit door and whispers, "psst, turn left at Belgium, mate".