After starting GSX with Ctrl + F12 I started Boarding, but nothing happens ( I can only see the cars ).
Please, describe the problem more precisely.
You say you can start boarding, so this seems to indicate GSX is running normally. But then you only see the cars. Which ones ? The ones provided by GSX, so the issue is really you are missing SOME GSX vehicles, or you see default FSX vehicles, so nothing from GSX is visible, and the program just stopped ?
In any case, are you using the latest version ? We had a very large update recently so, it's best if you use that one, so we are sure which version you have installed.