GPS has a spreading gain of 20,000 times. A 'good ' receiver, even if direct conversion, won't radiate more than 2 nanowatts IF it meets reasonable standards. The GPS signal is a long way down on that. The problem comes with anything that doesn't meet reasonable standards for radiation, and there's a lot of CE marked stuff where the CE stands for Chinese Export. A good professional GPS should never be a problem - it's the cheapo cheapo that might be. Regrettably, we have a whole load of European rules on these matters that aren't enforced anywhere except Germany.
There have been some documented cases in the US where the use of portable VHF FM broadcast receivers on airliners has interfered with the VOR because of the relatively high levels of local oscillator radiation allowed. There was a very good article some years back in the IEEE Spectrum magazine on that.