Unfortunately, GSX Jetways are not transparent
The *DEFAULT* replacement jetway is solid, because it was made to be similar in style and dimensions to the default model that was replaced. But there are plenty of transparent jetways in GSX you can choose from, when customizing an airport.
I still prefer using REX worldwide Jetways and hope GSX will find a way to accommodate.
REX worldwide jetways are exactly the same as the default ones, just with replaced textures so, even the one that "looks" like a glass version, it's not really transparent. It's just as solid as the default one so, clearly, there's no way we can let you see passengers through a solid jetway.