First, the ones you call "cargo containers", are ULD and they are used to load luggage on passenger airplanes as well:
https://en.wikipedia.org/wiki/Unit_load_deviceA unit load device (ULD) is a pallet or container used to load luggage, freight, and mail
But of course, GSX being the flexible program that is, allows to configure your airplane not to use them, in case you think they are not common in your area or airline.
In the airplane configuration editor, for each cargo door, in the "ULD Code" field, you can select one of the following options:
1) "Auto" - selects a conveyor belt or a cargo loader depending on the door's height.
2) "Belt" - forces a conveyor belt loader
3) an ULD code from the list - force a cargo loader, and will use the ULD listed here
And not only that. GSX will ALSO allow you to specific the use of a cargo loader or a conveyor belt depending on the scenery and/or parking spot, because this customization is also possible on each parking spot, so you can even specify that some airports will use belt loaders, other will use cargo loaders, and even some areas of the airport might use different loaders.