This option has been there for more than one year, and there are many 3rd party airplane already included that have cargo being loaded from the left side, for example the Majestic Dash8, but also several models from Feelthere, including the ERJ 135.
If your airplane is not internally supported, you can do it yourself using the airplane configuration editor that comes with GSX, by moving one cargo door to the left side AND rotate it 180 degrees, so the green arrow in the graphical editor will point to the outside.