The Q400 engines will start under ANY conditions except when GSX pushback is used. Sounds crazy I know, but don't see any other reason for it.
As already explained on the forum, GSX doesn't do ANYTHING to the Q400 and doesn't touch ANY of its variables, either standard or custom (we don't even know or care what they are).
The only thing that GSX does when pushing an airplane, is to send the standard Simconnect FREEZE events and the SIM DISABLED variable to the airplane when pushback starts, and the opposite "Unfreeze" and "re-enable" command when Pushback ends. Whatever internal simulation stops working when the simulation is disabled, is all happening inside the Q400, because everything works on an airplane that doesn't use a custom flight model or custom engine model.
In addition to that, GSX also sets a custom L: variable named FSDT_VAR_Frozen, that any other add-on can read, to know if it's being pushed by GSX, we added explicitly for the benefit of the Q400 developers, just because it might be easier to check than the other two.
So, they can check if GSX is pushing in several different ways, so they have all the means to behave like any other FSX airplane, in relationship to GSX. It's up to them to act on our support, since it's there since a year by now, we can't do anything more from our side.