You should really read the Paintkit documentation, even if you are not interested to creating your own repaint, because it describes in greater detail compared to the GSX normal manual, how the parking scoring system works.
It's not enough to assign an airline to the parking, before an airline can even get a chance to go there, it must be assigned to the airport first, and this is done by adding the ICAO code of the airport in each of the GSX vehicles SIM.CFG files.
In your case, since Alitalia has probably a score of 2 at LIPH (since its vehicles have a LI specification), it would be enough to add LIPH to Ryanair, without having to edit any parkings.
You need to edit parkings if there's a TIE, between two or more operators with a score of 4 (full ICAO code matched), in THAT case, the parking code will give a preference boost to the companies that match the parking.