That variable, like all gate-related services, depends on GSX knowing which gate you are on, and this check is not made until you either open the GSX menu when you are on a gate with brakes set and engines off OR if you start the sim already in that condition.
If you start the sim (or restart GSX ) with either the brakes off or the engines on, even if you are on a gate, GSX still haven't checked which gate you are on so, every variable related to the gate you are on won't be initialized. Basically, if you see the GSX Traffic Cones appearing around you (assuming they are not disabled for that particular plane), you know GSX has set a gate.
However, there's another L: Var you can check beforehand ( which I now realize we haven't documented it ), called GSX_STATE, which tells you the overall state GSX is in:
0 = FLYING
in flight but under 10K feet/250 kts, so GSX can still be called, to preselect a gate
1 = ON_GROUND_DISABLED_BY_USER
on ground, but GSX has been disabled on this airport, by setting its ICAO in the couatladdons.ini
2 = ON_GROUND_NO_SERVICE = 2
on ground, but outside a serviceable airport
3 = ON_GROUND_FREE_TAXYING = 3
on ground, outside a parking, not using a FollowMe
4 = ON_GROUND_ASSISTED_TAXYING = 4
on ground, outside a parking, using a FollowMe
5 = ON_GROUND_PARKED = 5
on ground, parked - this is the state you might be interested in
6 = ON_GROUND_PUSHBACK = 6
on ground, being pushed by GSX
7 = ON_GROUND_DISABLE_UI = 7
on ground, with the menu temporarily disabled, this usually last only a few seconds only to prevent calling the menu twice while vehicles are being created.
8 = FLYING_SELECTING_GATE = 8
in flight, same as a above, to prevent calling the menu twice while in flight and a gate has been pre-selected
9 = FLYING_HIGH = 9
in flight, over 10K feet or faster than 250kts, so GSX is not available to call.
