I have been unable to change the above aircraft to loaders and containers for cargo.
You are not supposed to change anything. The PMDG 777-200LR is internally supported by GSX, so it doesn't require ANY configuration, and by default it's configured to use only cargo loaders.
The only possible reason why you would get belt loaders, is that you made some kind of mistake trying to configure the airplane that didn't require any configuration to begin with.
Use the Reset button in the airplane configuration page, so GSX will revert to its own internal data, which surely works.
Also, be sure you haven't placed a GSX.CFG file that you might have downloaded (maybe made before GSX supported this plane internally), in the airplane folder. If you have it, you must removed manually, because the Reset button won't remove it (since that location is *supposed* to be used only by airplane developers to supply a GSX config for their plane that won't be reset by mistake)