PS: I already modified the gate vehicles positions but there is no option for the catering vehicles, for example.
That's because they don't spawn at the user parking, but on parking spots of the "Vehicle" or the "Fuel" kind, assuming the scenery have them, and assuming they are connected to the rest of the airport layout with proper paths of the "Vehicle" kind.
If the scenery AFCAD is not made well enough, GSX must find some way to reach your airplane, so vehicles might start from any parking ( if both "Vehicle" or "Fuel" parking spots are missing or unconnected ), and will take any possible route to get there, including passing over taxiways and even crossing runways, if there's NO other choice.
And, lacking a proper and realistic connection between vehicle paths, it's likely those vehicle might take strange routes, especially when reaching the airplane, because they *must* reach you in any possible way.
So, basically, it's the old issue of "Trash IN = Trash OUT".
A well designed AFCAD, will go to great lengths to result in a smoother ground movement. That's the whole reason why such dedicated vehicle paths exists in real life too.