Oops! Just checked and I already have the June 2013 livery installed. So that can't be the problem. I will download and install latest version of the main program, however.
That doesn't mean much: that file is *constantly* updated, because it contains *every* vehicle released AFTER that date. It doesn't mean that, if you install it once, you don't need it anymore!
The problem only occurs when the stair unit with the canopy is deployed. The one without the canopy attaches at the correct door height. What controls which stair unit is deployed? I have never had issues with other aircraft models and GSX, only the PMDG NGX.
The problem happens if you have outdated vehicles. As I've said, that specific staircase has been updated at some point, and won't work unless you update the vehicles.
Of course, installing the current GSX full installer will have the same effect, but it would have been exactly the same as if you just downloaded one of the vehicle updates, the smallest one is usually enough, if your GSX install is not too old, but any of the 3 will work just the same, because each one contains *everything* released AFTER its indicated date, up to today. And so does the GSX Full installer so, whatever of the 4 installer you will use, it will fix this problem.