You must destroy as many monsters as possible based on the number of Heads you flip. If you flip 2 Heads, and the only monsters on the field are 1 of your opponent’s monsters and your “Gatling Dragon”, you must destroy both monsters. If there are only 2 monsters on the field and you flip 3 Heads, destroy both monsters.
You cannot activate “My Body as a Shield” as a chain to “Gatling Dragon” because it has not yet been decided if any monster(s) will be destroyed or not.