Installing GSX replaces the original jetway model ( the one used by DEFAULT airports ) with a GSX/SODE version, which means all default airports will have their jetways automatically upgraded to GSX.
However, any 3rd party scenery that used CTRL+J jetways with a custom model wouldn't be affected in any way, until you to through the process of creating an Exclude file and customize the scenery with GSX.
If you want to restore the original model, so *default* airports will go back to the CTRL+J logic, you use the GSX Control Panel to Restore default jetways.
It's impossible to keep the GSX logic with the default model active, since in order to use GSX, a jetway must follow the SODE standard. However, it seems strange you would want to achieve this, considering the model GSX use to replace the default model it's perfectly compatible in size with the original one ( so it will fit the default airports as the original one ), just looking much better.
3rd party sceneries will always be unaffected by any of this so, I'm not sure why you'd want to do it, the option to restore default jetways is there mostly for one situation: you installed GSX, but are not interested in the Level 2 features but, since L2 works only in Trial airports unless it's purchased, if you don't want L2 features, with the Restore option you can restore the default jetways at default airports to their original function, since by default, after installing GSX, unless L2 is purchased, default jetways at default airports outside the Trial ones would become static.
