What can be wrong?
That staircase can reach up to 6.093 mt., and the door's height on the 777 are 5.52 mt. so there's no reason why it couldn't reach it, also because the doors is configured correctly from your screenshot.
The only possible explanation, is that you don't have the last version, you must have installed from an old full installer, because that staircase has been updated to reach higher somewhere in Sept. 2013, and is surely included in all installers released after that date.
Download the current GSX Full installer and be sure you install from that one. Also, be sure you don't have objects from an old installation of the PaintKit, if yes, you must update that one too, because the regular GSX installer won't update anything coming from the PaintKit.