Before you can use any airplane on GSX, it must be configured first.
Some popular 3rd party airplane add-ons are already included in the internal GSX database but, of course, we cannot include every airplane, but that's not a problem, since GSX comes with a fully-featured airplane configuration editor, which you are supposed to use when you use an unsupported airplane for the first time.
In general, to see passengers, you must set all doors correctly.