This would be the job of the Living World Config file, it controls what vehicles are used for various purposes. The Services.xml controls which service scripts are available and used for what.
Are you trying to change the services at different gates at the airport, or just applying the same rules to all gates at the airport?
To make it gate specific, I think you can control which services are used based on the type of gate it is, but I haven’t done that myself yet. So, in other words, a GATE_SMALL can be different from a GATE_HEAVY. You’d have to research the correct terminology to do this though.