Anyways, whenever my Flight Sim "regenerates airport cache" it removes all the ground handlers, giving me randome ones.
It surely doesn't do that. You don't say from *where* the ground handlers are removed but, assuming you mean they have been removed from the GSX parking customization page, this will not happen "just" because the sim regenerated the cache, but because it regenerated the cache because the AFCAD for THAT specific airport is different, and in that case it's entirely normal.
GSX associates a scenery customization file with the AFCAD it was created with, so you can have multiple GSX .INI files, depending on which AFCAD they were based on, since their parkings usually don't match, so it would have been wrong if GSX tried to have a general customization file for that airport ICAO but, instead, it correctly use a different .INI file for each AFCAD you might have used at that airport, matching it by name.
So, if you change the AFCAD with one with a different name, a new customization file will be used, so you'll have to edit that one too.
Say if I want Menzies, Swissport, Aviator or DNATA to do the groundhandling, what 3 letter code do I put into GSX?
The 3 letter code is used ONLY if you want to MATCH an existing 3 letter code in the scenery AFCAD with the GSX parking. That's because the AFCAD standard supports only 3 letter codes, so that's the only thing you should enter there, in THAT specific case.
The reason for wanting to match an existing parking code in the scenery AFCAD, is to give it a preference on such parkings, compared to other other operators with the same score across the whole airport.
But that doesn't mean you are restricted to 3 letter codes only! GSX itself is not limited to 3 letter codes, and there are many ground handlers included in GSX that are NOT airlines, so they don't have a 3 letter ICAO code, so they have different and usually longer codes.
So, DNATA it's just DNATA, Swissport it's _SWP, etc. You can see their actual code by opening the SIM.CFG of any ground handling vehicle, such as the FSDT_Staircase, for example, and check the Couatl.atc_parking_codes lines for each variation.
Also, is there a way to change the cathering veichles? They seem to be completely random all the time. I get german caterting veichles in Dubai, where it should be skychefs or something.
There's no way to do it from the GSX parking customization page, like the ground handlers, but we might add it in future versions. Right now, it's only possible by directly editing their SIM.CFG, according to the explanations in the PaintKit. And no, they aren't obviously random, but haven't been customized by default so extensively as the ground handlers are.
