The definition for an "Act of God" varies from state to state and then from one insurance company to another. A good general definition of this phrase though is given by the Maryland Insurance Administration (MIA) site which states that an Act of God is a natural occurrence beyond human control or influence. Such acts of nature include hurricanes, earthquakes, and floods.
As an example the MD insurance regulator website notes that a snow storm is an act of God but driving in a snow storm is an act of man or woman or teenager. An Act of God is defined by most of the insurance industry as a natural occurrence beyond human control or influence.
In your case while the wind itself may be considered an Act of God, opening a car door is an act of a person and thus should be under the person's control. If the wind was so strong that it bent a tree over as to snap and hit your car or another car than they would be considered an Act of God or Act of Nature as some also call it.
So in general the situation you described would be considered your fault and not an Act of God since you failed to control the car door. Though in strong winds it can be difficult to control a car door, if it gets away liability laws usually see this as negligence on your part, thus leaving you liable.
An Act of God or an Act of Nature basically refers to a natural event, not preventable by any human agency, such as flood, storms, or lightning. These forces of nature a person has no control over, and therefore cannot be held accountable but opening a door while it is windy out is an act of a person and thus would found to be your fault normally.
Since you could be held at fault than the car owner of the damaged car could place a claim against your Property Damage Liability coverage for the cost of the repairs due to your car door hitting their vehicle, if that is what happened in your situation. If the wind took and bent your door so that it needs to be fixed than this would usually fall under your comprehensive coverage.
To find out your insurer's definition of an Act of God speak with your agent. And to see if your state has a definition for this phrase contact your state's insurance regulator.