could you please advice on what conditions do i get an option to select the ground handling services?
When there are more than 1 candidate with the same score at that parking on that specific airport.
and on what condition gsx selects a random ground handling services?
Normally, it shouldn't. But there might be a condition when this could happen:
In latest versions, we added the distinction between cargo and non-cargo companies, so you won't have unrealistic situations of having passenger companies on cargo parkings.
However, there's a new fad going around with some developers, that flags *everything* as cargo or military cargo, most likely to get rid of default ground vehicles, something that might have been achieved using the "ga ramp" type, in order to preserve the difference between cargo and non-cargo. In this situation, you'll probably get random operators.
Which is why, we added also the ability to override the parking TYPE too, so you can restore the proper parking type, without having to edit the scenery AFCAD.