The current Code-500 Ephem only supports J2000 coordinate systems, but Steve S reports that the FDF uses other formats as well:
According to the Code500 spec, you can have True-of-Date/Reference, B1950, J2000, or Earth-fixed. Of those we never use B1950 or True-of-Reference. For reading and writing Code500 ephems, GMAT should support True-of-Date, J2000, and Earth-fixed.
The referenced spec is attached.
The attached script will throw an error attempting to generate a Code-500 ephemeris in EarthFixed coordinates.