I am starting to suspect this may be a GSX problem and not a Simbrief problem. I am trying to load up my iniBuilds A300 pax variant and GSX is reading the aircraft as the default MSFS Wright Flyer. I have had the same issue with the PMDG DC-6B (passenger variant) just last week. I was able to tweak the GSX profile to get it to work, but I am at a loss with the A300 profile. I know the A300 pax and freighter variant work with GSX because I fly them quite regularly and out of the blue, they no longer work.
The custom GSX profile that ships with the iniBuilds A300 has been placed in it's respected GSX/Airplanes folder, but GSX will not read it. The ICAO code is correct, being A306, both in Simbrief and the GSX.ini file. Regarding the DC-6 issue, I had to change it from DC6B to just DC6. But I changing the ICAO doesn't fix the A300 file.
According to the Customize Aircraft menu, it says the Aircraft data used by GSX is the Aircraft.cfg, which obviously it's not supposed to do, and making changes to said file will break the airplane.
Attached are a screenshot of the aircraft menu, as well as the custom GSX profile provided by iniBuilds.