I can't reproduce the problem.
- With the CJ4, all works normally, I made a SimBrief plan and GSX recognize it correctly, see the first screenshot.
- With the Longitude, it seems there's a bug in the airplane.cfg, the icao_type_designator is set as "C25C" instead of C700 as it should be but, even in this case, GSX correctly reports the error of a non-matching airplane, see the 2nd screenshot.