Transforms are composed of two parts. A Vector3 called origin, representing the translation of your Spatial, and a Basis called basis, representing rotation and scale. Instead of only sending global_transform.origin, you could send the Transform directly. Either make your own function that sets the transform property on the other end, or use rset().
Note that you probably want to use transform instead of global_transform.
rpc_unreliable("set", ["transform", transform])