The Grad is artillery you need to create a fire mission and execute it.
Here is a simple example:
// There is a arty logic ingame named arty1;
_myBattery = arty1;
// We need to make a arty fire mission template
_heTemplate = ["IMMEDIATE", "HE", 0, 15];
// We need the postion of the target Above Sea Level.
_targetPos = getPosASL target_1;
// Here we check and make sure it is range then we exicute the fire mission.
if ([_myBattery, _targetPos, _heTemplate select 1] call BIS_ARTY_F_PosInRange) then
[_myBattery, _targetPos, _heTemplate] call BIS_ARTY_F_ExecuteTemplateMission;
hint "Firing... Sit back and wait, it will take about 60-90 seconds for the rounds to impact";
hint "Target out of range!";
Use the radio trigger in the mission to fire the arty.