[MikeS (OP)]

GPE relative GP huh

To find GPE you need a mass, the gravitational potential energy is the energy associated with the position of a given mass in the gravitational field. To find GP you also need a test particle of rest mass to define it from. The Principle of Equivalence when it compares Earths gravity to a rocket accelerating uniformly constantly at one gravity also uses a 'test particle', aka that rocket. Without the 'test particle' there can be no descriptions of a gravitational field as you have nothing to describe the field against as it seems to me.

Fields and relative motion gives me constant headaches.

Not necessarily so.
We weigh less on the Earth than on Jupiter and more than on the Mars because Jupiter's gravitational field is stronger that the Earth's and Earth's gravitational field is stronger than that of the Mars.  That is a direct indication of the differences in the gravitational field.  Our clocks on all three planets would indicate time passing at the same rate but relative to a distant observer they would all be running at different rates.  This is an indirect indication of the differences in the gravitational field.  No test particle required.

[yor_on]

I'm not questioning that Mike Fields vary, and the way to define that variation is to measure. Measuring though always will involve some 'test particle/detector' or similar from where you define the variations. It would be really nice to measure without one but that can only be done theoretically using what we measured already via detectors. But it is also so that your 'frame of reference' measuring will define what you measure, as 'free falling' in a gravitational field moving uniformly/gravitationally 'accelerating', relative accelerating locally. And that gores for all types of fields I can think of now (measurable ones that is)

[yor_on]

The point is the question what we mean by 'relative motion' here. If everything is exchangeable except locally defined accelerations then what you see and measure becomes a local result, dependent on your 'frame of reference', and that means locally defined here. I like it as it fits my 'strictly local' point of view, but it also gives me a headache when looking at what we think we see, 'SpaceTime'.