paulo ... as stated above its the not knowing there maybe a risk of interference - if you ve asked or there is equipment fitted the commander will be aware of the potential problem... And I guess any installed equipment would have to be tested and proved to be immune from causing interference.
Also consider this: A GPS causing nav aids/compass to be a degree or two out in your light aircraft probably won't cause any noticable problems over the ususal distances flown especially VFR, but compare than to one or two degrees over 1000s of miles..... 1 in 60 rule?
Just as an example... in a strong signal area where you mobile is operating at low power make a landline call with the mobile in your other hand and see what happens. Then imagine whats happening to comms, etc in the air when the mobile is operating at high power trying to lock onto a cell site.