The issue with Steam DLC has been discussed already in the forum, and there's just nothing we can do to fix this.
Such products use the Steam DRM and have encrypted all their files, meaning it's not possible to open their .BGL AFCAD file to read data about the airport, so GSX will only use the default gates.
Of course, this won't affect just GSX, but any other utility that will have to open the AFCAD file, like flight planners or editors like ADE, for example, making these addons unusable with such utilities.
Your only option is to contact the developers, and ask if they could supply you with an unencrypted version of at least the AFCAD file.