I have tried without luck to edit the sim.cfg file of the cargo loader.
The Cargo loader supplied with GSX is the FMC Commander 15 model, and its platform can go from 1.92 to 3.55 meters. This is 100% accurate to the actual specifications of the real vehicle.
You can't just change these parameters in the loader Sim.cfg, because they must match the physical model capabilities and how it's animated and modeled. In order to reach higher door heights, as in real life, we would need a different loader model.
The only way to not to have your cargo floating, with the current model, is to be sure no door is higher than the maximum reachable height of the FMC 15, which is 3.55 meters. This is the only possible solution, until we'll add an entirely new model that can reach higher than that.