The configuration that comes with GSX for the PMDG 737-800 (only the 800, both regular and WL versions), use the Conditional Embedded stair feature, meaning it will use or not the embedded staircase depending if the airplane variable that controls its appearance has been set by the airplane so, GSX won't mistakenly think you have an Embedded stair when you don't, unless:
- The airplane hasn't set its own custom variable for some reason. It's called L:NGXAirStairs
OR
- You made the configuration for the Embedded staircase. This is not required, since the configuration that comes with GSX already has it. In fact, it's possible that, by setting the option yourself, you might have caused the issue because, while the GSX editor has the ability to create a normal Embedded staircase with no conditions, it doesn't have the user interface to add a Condition so, even if the manual explains it's possible, it's only possible with the internal GSX configuration that has some features end users still cannot access but, since your own customization always takes precedence over the internal one, by creating an Embedded staircase, you might have overwritten the one in the default GSX configuration, that has a Condition, precisely to prevent what you are reporting.
So, first thing to do, Reset the airplane configuration, and use the one we supply with GSX for the PMDG 737-800