there comes in the green line "Remember to operate the jetways", I don't know what to do? What does this mean?
Since, as clearly explained on the manual (highlighted in a blue box) and as discussed many times already on the forum, GSX doesn't control or change in ANY way how jetways are operated, it's simply reminding you to operate it yourself, using whatever means the scenery you use has to operate (or not) its jetways.
I always do nothing, and then there aren't the jetways...
If you don't do nothing, nothing will happen...which is why GSX told you to do it, because it doesn't do it itself.
and the passengeres are (de-)boarded WITHOUT the jetways, and this is NOT realistic...
You can't fault GSX if you decided to ignore what it told you to do. There are several reason why GSX doesn't operate jetways automatically, the first one is that is not possible to ask FSX about the jetway current status and since the key to operate it is a *toggle*, there's no way to be sure it's being set correctly, if done automatically.
The other reason is that, if you use a 3rd party scenery with static jetways, but you correctly configure GSX to tell that parking HAS a jetway (although a static one), it needs to know about it, because otherwise it will show front passengers stairs, but it can't obviously turn a static jetway into an animated one. And, there are a couple of 3rd party sceneries that have non-standard way of operating them, like using Com radio frequencies.
That's why the only sensible option is to let the user control the jetway, assuming it IS controllable in the first place, which is true for all default airports, all FSDT airports, and some airports from Flightbeam and UK2000, using the standard FSX key command for jetways, which is CTRL+J
Am I able to get out of the plane? I saw this in a few videos...
Using 3rd party camera-controle addons, like EZdok camera.