As explained many times on the forum, and of course on the manual, before being able to use an unsupported airplane, you must create a configuration for it, using the airplane configuration utility that comes with GSX.
This process, in addition to positioning the doors, requires to tell which variable holds the status of a specific door.
So, you must first figure it out which variable controls that door on that specific airplane, it might be one of the 4 default variables, so in that case you just try with all of them, or it might be a custom variable, in that case you'll have to write an XML expression that checks for that variable.
Figuring out the name of the variable might be a bit tricky, but it's usually not difficult using an hex editor over the airplane .MDL file. Most developers, when they use a custom variable, the name usually contains the "door" string, and searching for it with an hex editor usually gives good results.
If this sounds too complex for you, GSX obviously gives you the option to Ignore the door (which is NOT the same as disabling it), so the service will continue without checking if the door is open or closed, but then you must remember to open or close it, otherwise you'll see things like the catering crew or luggages passing through the airplane bulkheads.
