First, I suggest to read the manual at Page 36, which explains how to do this.
2. I edited the handling rules file in the following route C:\Program Files (x86)\Addon Manager\texture\rules_handling.cfg
This is not the proper way of doing it, because each time you'll run a GSX Live Update, that file will go back to default, so you'll lose all your modifications.
If you followed the standard procedure to add a custom logo, GSX would create a new
rules.cfg file for that operator folder under %APPDATA%\virtuali\GSX\texture.
By editing this file, you can create your own rules that won't be affected by the FSDT Live Update. In addition to that, it will also create a "blank" ( except with some comments at the start, which are supposed to read... )
rules_handling.cfg file in the %APPDATA%\virtuali\GSX\texture root folder, which can be used to create rules that override the default rule of all STOCK operators that comes with GSX, again without being affected by the FSDT Live Update.
This because, it might be possible that, if you are trying to add your custom operator on an airport, that airport might already have a stock GSX operator assigned to with the same priority, so it might still appear and this might not be what you want, so you can "downgrade" or even remove a default operator from that airport, to make room for your own.
3. I edited the fltsim line in the aircraft.cfg file of the airplane I'm using with my own VA livery, in this cases FSLabs. I added atc_parking_codes of my VA (Attached picture)
This is why it didn't work.
As explained at Page 25 of the GSX manual, what drives the choice of the ground handling operator is the atc_parking_codes of the PARKING spot, NOT the airplane! And this is, incidentally, for the precise reason you said you don't want to have a ground operator "following you everywhere", which is precisely what will happen if GSX used the airplane codes.
Instead, as also explained on the GSX manual, at Page 64, the atc_parking_codes of the AIRPLANE are used only to select the ULD containers logos and codes. Because, opposite to the ground handlers, you WANT to have your packages always being the same, and not being influenced by the airport. If the aircraft.cfg doesn't contain any codes, the ULD will default to using the airport ground handler.
