These doors do not open with a keyboard button, they have a special door control gauge section that gets me to push the "Door Button".
How you open doors doesn't make any difference to GSX, it doesn't check key presses, it only checks the internal FSX simulation variable that holds the doors status, which will of course change if the door is open/closed, regardless how you do it.
So, in order to properly configure an unsupported airplane, you must figure out how the developer mapped the FSX door variables and tell GSX. If they used the official FSX variables, they are fairly easy to figure out (there are only 4 of them), it might be more difficult, but still doable if the developers used custom L: variables, in this case you'll have to write an XML expression to check a custom variable. This is explained in the GSX manual, Pages 28-29