The default configuration for the PMDG 777 that comes with GSX, is configured to use cargo loaders so, you don't have to do anything to see them.
If you are not seeing the cargo loaders, the only possible explanation is that you tried to configure the airplane yourself, which wasn't needed since it's already internally supported in GSX, made some kind of mistake, so now your customization is taking over the one that comes with GSX.
The airplane configuration editor will tell you from which source GSX is taking its data from so, if it's not saying it's taking the data from the internal GSX database, you should Reset to undo all your changes and use the default that comes with GSX.
And note that, if you downloaded a GSX.CFG from some place, and put it into the main airplane folder, this won't be reset with the "Reset" button, because that's not the correct place for user customizations, but it's a location reserved for airplane developers, that are supposed to supply their plane with a configuration that works so, it's not removed but, if you placed a downloaded customization there (again, not needed because GSX is already pre-configured for the PMDG 777), it will take precedence over the GSX internal data so, if it's wrong, it will cause any kind of problems. In this case, the GSX.CFG file in the airplane folder should be manually removed.