I can confirm disabling the staircases works just fine, I just checked it, and it works regardless if the parking spot has a jetway or not. Are you sure the vehicle you are seeing is coming from GSX ?
Also, is it happening only at that airport ? Have you checked GSX loaded your profile ?
Note that, if you are editing a GSX custom profile which was originally made for P3D V4 and contained custom operator textures, since this feature is not available in P3D V5, as soon as you edit such file, GSX will create a new copy, with a "p3d5only = 1" flag, so P3D V4 will continue to use the original file, and V5 the new one.