The PMDG 748-800 is included in the GSX internal database, so it's working without having to do anything. However, if you downloaded or made a configuration for it, and this is wrong, anything can happen, because user-made configurations have an higher precedence over the GSX internal ones.
To check if you have a user configuration, open the GSX airplane configuration editor, and check if it says "internal data". If it's not, then you are using a user configuration you either made or downloaded, and this is likely wrong. In this case, press the Reset button, so GSX will revert to its own internal configuration, which surely works.
No, the nose cargo is not supported by GSX.
