I just installed FSLTL and made two tests, one without the Traffic Injection program, and another with it. In both cases, I can't see any missing vehicles or animations in GSX,
See the attached screenshots, at default Milan Malpensa airport ( LIMC ), the first one is with no Traffic Injection, the second is with it. I used all default settings, and set default traffic to Off and default ground aircraft density to 0.
Here's a link to a complete flight using 3rd party airports, 3rd party plane GSX and FSLTL, no issues whatsoever both on departure and arrival: