No, the shown addon scenery uses the standard jetways without additional ground markings or additional ground polygons.
Then they aren't the default jetways, unless they have been replaced by the scenery itself or by another add-on that replaced them. I can only repeat and confirm the default jetway model, which has the same GUID you indicated DOES have ground marking so, the only possible explanation left is, either that scenery or some other add-on must have replaced it.
Where can I read how the jetways or the ground markings are changed in the GSX customization?
If you haven't changed anything in GSX scenery customization page, you can be sure the default jetway has been replaced by GSX with a model that have the same dimension of the default one, and comes with an integrated bridge and ground markings that cannot be disabled.
Which, again, can be changed, and in every other model ground markings CAN be disabled optionally. There are a couple of video tutorials in this forum, if you find the explanation in the manual not clear enough.