You said "no matter the airport", but it would be best if you said the airport because, the most common cause for this problem, is not Disabling the GSX Jetway Replacement files for the airport you have a 3rd party add-on for because, as explained on the manual, those are only supposed to be used in DEFAULT airports, they MUST be disabled in 3rd party airports.
- Installing GSX will automatically exclude 3rd party airports which can be recognized as such. As the manual says, only the ones matching the official naming standard will be recognized automatically, the ones who don't must be excluded manually by their code.
- Updating GSX will also do the same as above.
- If you install a new scenery after GSX and don't update GSX, you must still go again in the Config page of the FSDT installer, to check again for 3rd party airports to be excluded, and eventually manually add the ones that don't follow the official naming convention ( freeware usually has this problem )