I know the Majestic DH8D is slightly different in the way it interacts with FSX by the nature of the modelling. For the first time today GSX gave me the option to start engines on the stand before pushback - is this new???
Yes. Requested by users, and added now...
Anyway I started on the stand - how do I tell GSX they are started and I am ready for pushback?
On any FSX airplane, you don't have to do anything, since GSX will know if engines are turned on.
The Majestic Dash8 doesn't work as with any other FSX plane, so the standard FSX internal variables that indicates if an engine is turned on might not work. Until today, users of that airplane were able to indicate GSX that engines are off (when parking), by pressing the CTRL+Shift+F1 (shut down fuel pumps), so you might try use the matching reverse command (CTRL+Shift+F4), but I'm not sure it will be enough.