How to Measure Basical Velocity in General Relativity

The velocity of objects isn't well defined in g r because it requires an arbitrary choice. So you and i could be right next to each other, looking at the same photon, but if you have a different definition of basical velocity than i do, we could get different answers. That's why people say it's not well defined, because there's an infinite number of arbitrary choices you could make that do effect your answer.