If the start point of the arc is equal to the end point, the arc is a full circle of 360°.
The arc definition has a fuzziness of the order of magnitude of the arc deviation. The writer of the Gerber file accepts any interpretation within the fuzziness above as valid. If the writer requires a more precise interpretation of the arc he needs to write arcs with lower deviation.
Y199995D01* <---- Arc Start Point
G03Y199994I81851D01* <----- Arc End Point "with same X" and only 000001 difference in Y.