Before I had Brussels airport installed, the jetways were in glass. No messages from GSX like " this gate has a default jetway.
Yes, you also had the message from GSX it has a default jetway because, in MSFS ALL jetways are default.
That message would change only with FSX/P3D if the jetway is SODE so, it might change in the future if we'll have an alternate Jetway animation system available but, for the time being, GSX in MSFS will always say a jetway is default, because all jetways in MSFS use the default animation system.
You just haven't noticed the message. And yes, if the jetways were glass and since you didn't have the Aerosoft Brussel installed, they were surely the GSX replacement jetways, which was the correct situation in that case. And the Asobo models are not glass.
I installed Brussels Airport, excluded the 3rth party in the config screen as needed. Started up my sim and the jetways were in metal
This just can't happen, and doesn't have anything to do with GSX because, even if you forgot to Disable the scenery from GSX, you should have seen the GSX jetways, which at EBBR ARE in Glass.
Also, according to the screenshots I see on Aerosoft site, EBBR should come with its own custom Jetways, which are in glass as well so, if you correctly disabled GSX replacement jetways, you should see the Aerosoft glass Jetways instead of the GSX glass jetways, which would be the correct situation in this case.
So, it SEEMS you are now seeing the Asobo jetways, but I can't be sure of that, because you haven't posted a screenshot, you just said they are metal.
The only thing you can be sure of, all GSX jetways at EBBR are glass so, Disabled or not, if you see metal Jetways at EBBR, it's completely unrelated to GSX and, assuming there are no Metal jetways on Aerosoft EBBR, it seems you might have another conflicting scenery, perhaps a freeware scenery for EBBR you haven't removed, for example.