You cannot assume that *every* airplane would have doors controlled by the standard doors variables. Many 3rd party airplanes either use a non-standard rearrangement of the standard doors variables, or they have custom XML variables.
GSX allows both cases, as explained in the manual when the airplane configuration editor is explained, but you must insert the correct values.
Or, if this is too difficult (maybe because the 3rd party airplane uses custom variables not easy to figure it out), you can just set GSX to IGNORE the door status, so it won't ask you to open doors anymore. But then, if you don't open them, GSX crew and luggage will happily pass through the airplane bulkheads, because the whole point of waiting for the doors to be opened, was precisely to prevent this.
