phat
"Your GPS points whatever direction was true north at your lat/long when they programmed your software. It can be years out of date.. doesn't matter too much - but trust me, you're at the mercy of some bozo who coded up the declination."

I'm not sure of that Bob. I am about 100% sure that there is no magnetic flux gate in the GPS so it can only point to "grid north" unless its told to find "magnetic north, in which case it could be off for the reasons you state. It cannot sense magnetism.
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.