When using apierror.FromError with an http error, the status field does not get populated, so the GRPCStatus() method returns nil. It seems like this is intended to mean "no status is available". However, the status package treats nil as "OK":
https://github.com/grpc/grpc-go/blob/dba26e15a07f43875ccf806a2dd6cbcbc1c12eab/internal/status/status.go#L72

The status package also says that no non-nil error should have status of "OK".
To properly fulfil this contract, it seems that the APIError.status should never be nil. I would expect it to at least return UNKNOWN, but it looks like the json error may also include a status field with a grpc status code, and some of the http status code seem to map directly to grpc codes (e.g. 404, 429, 503):
https://cloud.google.com/apis/design/errors#handling_errors