Jetways won't work unless they are associated with a parking, that's why you are required to exclude 3rd party jetways, because the replacement .BGLs that come with GSX are only meant to be used with default airports, since they contain only jetways, associated with the same parking they had in the default airport.
They only way to add GSX jetways on a 3rd party airport (unless you made the scenery yourself, so you have the project source), is to create a new Airport project with the MSFS DevMode, which should contain:
- An exclusion rectangle covering the whole airport, with a command to exclude only jetways.
- A set of parking spots ( at least those with a jetway ), matching exactly the name/number/suffix of the original scenery
- GSX Jetways, which can be chosen freely in the entire catalog of models added by GSX, associated to those parking spots.
Of course, if you are the developer of the original scenery, you can just replace the jetways used in the scenery with GSX ones, and recompile the scenery, so it's likely that after freeware GSX profiles, we should see freeware airports using GSX jetways individually placed.