I am not sure if the customization of the parking alone is enough to get your GSX running. I think the Problems are the taxyroutes (or car routes) are missing in your AFCAD. (I am no expert, just a guess). So you can customize as long as you want, but the car's still can not come to that place. (It's like a model railway, missing tracks). So you should look at the AFCAD in the free AFCAD Editor (there is a topic about). There you have to add the taxyroutes, then it should work. But don't ask me how, i never dared to touch that yet...