well, i think this will work put the line of code outside of the cfg vehicle class
   class CfgVehicleActions
{
   ACEGunner="CombatReloadMortarStart";
};
then find the class name of the animation that you want and put it where
CombatReloadMortarStart
is, then in the cfg vehicles part put this line of code in
gunnerAction="ManActACEGunner";
it is important that you keep the ManAct part