You may use a GPS on your flight. In fact Garmin encourages you to do so 'cause you can play with it during that time, makes it much more fun to get to know your unit.
Have had mine on every flight so far, no issues, with the exception of a DC-9 crew who wanted me to leave it up front for them
While it is a receiver, it is an aviation approved receiver, and as to your situation, I simply tell you to do this:
Bottom line: Before flight, simply ask the Captain, if he says yes, you're done. Use it and be happy!
Most airlines give the captain discretion over what is and is not allowed, and they typically will go by the airlines policy.
As an ex Naval Avionics technician as well, I go with Paulo's answer. In fact I worked with Hardened aircraft for EMP, and to tell you, the port GPS in the back isn't generating enough RF field, if at all, to interfere. There have been larger problems with in flight fires from the entertainment systems onboard that have shown some degree of interferance than your unit will.
On the other side, knowing your position can be aggravating, I went from FL to Jersey, and held outside West Virginia for 3 laps, then over DC we diverted, and you could tell we were diverting by the course, and looking out at the weather ahead, my FA thought it was great knowing ahead of time, as when the captain gave us the divert speech, we plugged in direct to the divert location, and had an instant idea of how long to get there (for the most part, going around the klag, Newark was closed hehe) ...