If I follow the code loop I end up with white or bare as logical choices. When you hang your amp probe on a GEC it will usually be carrying some of the unbalanced load.
It can be seen as an extension of the grounded conductor from the utility in a ring circuit that ends up being a parallel path back to the X0 via the earth or the water system.
It is hard to get over the idea that "grounding" conductors are not green but this is really a "grounded" conductor.
