I have 3rd party airport that has static jetways, I want to replace those with GSX's jetways
You shouldn't do that in the first place, because the GSX jetway replacement files are only meant for default airports, since jetways are placed in the same position as they default ones, in the default airport so, they won't match very well with a 3rd party airport that likely has a different terminal structure. But in addition to the visual problems, they will likely malfunction as well because, what makes a jetway working, is its association with a specific parking name/number, and the GSX replacement jetways are associated with parking names/numbers coming from the default scenery.
So, unless you have the very unlikely situation of a 3rd party scenery that has its own jetways placed exactly in the same position as the default ones from the default airport, and also have its parking spot named and numbered exactly like the default airport, the GSX replacement file shouldn't be used, and that's why GSX tries to automatically detect all 3rd party airports, and Disable the jetway replacement file for them automatically.