[apsophus]

What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case?

General Relativity may seem counterintuitive, but experimentally it really does seem to be the way our universe works. Starting out from the position that absolute motion is meaningless has proven remarkably fruitful. It's also something a lot of very smart people have been thinking about for a hundred years. Whether or not you find Mach's arguments plausible, they're fundamental philosphical claims and can't simply be dismissed through a half-baked thought experiment.

[thaumasiotes]

> What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case?

In that case, it wouldn't be drawing you outward no matter where you were located. So, spinning would be "you feel a gravitational force", and a ring of mass would be "you don't feel a gravitational force".

[jfhufl]

Seems to me the main difference is that all the particles in the gyro are trying to go in a straight line, but can't due to being a part of a rigid structure. You should be able to tell the difference by dropping a ball bearing. It will land "straight down" if it's gravity, but move "to the side" if it's rotating.