The GSX airline codes might be the same as the ones in ADE, but not always.
In this case, there was a conflict in ADE, because JMC is assigned to both JMC Airlines (an airline that doesn't exists anymore) and Jet2. And, the real world ICAO code for Jet2 is EXS but, EXS in ADE points to their former name, which was Channel Express Airways, now they are called Jet2.com.
So, in order to have GSX displaying the correct name, it uses Jet2.com for Jet2, and that's what you must insert as an airline code, either in a parking spot, or in the Virtual Airline override setting. If you put it as Virtual Airline override, it will take over *everywhere*, since this feature was made for use for Virtual Airlines, that might want to use their equipment whenever they go.
Most of the time, you want to set airline code for a single parking, a terminal, or the whole airport at most, so you use the Parking customization page at the affected airport, not the general settings page.
To know for sure which code is required by GSX, look at the GSX.CFG of any GSX vehicle, for example the Staircase, and find the lines beginning with Couatl.atc_parking_codes=, with the actual code following.