The iFly 747-400 is very new and it's not already internally supported by GSX. But of course, like any unsupported airplane, you can create a custom configuration for it, as explained in the GSX manual, so you can use GSX with it, without waiting to be supported.
If GSX keep asking for a door, it means you indicate its variable in the wrong way. If you can't figure it out, set it as "Door without open/close checks", so GSX won't wait for it.