It's not clear which Heathrow airport you are using.
- If it's the default Heathrow airport, GSX Level 2 would have replaced the jetway ( default system only allows for a single jetway ) automatically with a GSX jetway, which can work if you have a GSX Level 2 license. However, if you want to add other jetways on a parking spots, you must add them yourself, using the GSX parking customization page, and choose from many jetway models, which include also an high model suitable for an A380.
- If it's a 3rd party scenery that doesn't use the default jetway models, GSX hasn't changed anything to the jetways automatically, it will never do that on 3rd party sceneries on its own. It would be you that will have to go through a specific procedure, explained in the manual and on some video tutorials posted here in the forum, which require to first create an Exclude file to disable jetways the came with the airport, and then add the GSX ones, using the same method as above.
Of course, you must also create a proper configuration for the A380 you use, since it's not included in GSX by default, and set all doors correctly using the included Airplane configuration editor in GSX.