I fly solely British Airways out of LHR (occasionally LGW) and therefore am not wanting a generic service but a BA accurate one. BA use Gate Gormet for Long Haul operations and DHL for Short Haul. Currently there is a BA option on GSX when requesting ground support however it sends a variety of Catering Trucks to the aircraft, which isn't quite as accurate as i'd like.
I'm not sure I could possibly explain here in any better way that what is already explained on the PaintKit documentation.
Since there's no way, as far as GSX is concerned, to know if you are "short haul" or "long haul", the only possible way to do something like this, is to act on the "condition" parameters of all the variations of the catering vehicle SIM.CFG files (in their specific [fltsim] section for the affected liveries), and have the different liveries use different conditions to appear depending on the airplane service door's heights, which is something that might be used to identify a short haul plane vs a long haul one.
And, since every change you make to the default GSX vehicles will likely be overwritten by update vehicles installers and/or future GSX full installers so, you should also probably act on "copies" of Gate Gourmet vehicles in the *USER folders (those of the Paintkit), assign a 4 letters ICAO matching for them to the airports you are interested in, so they will be used by preference against the default ones that comes with GSX.