You probably have a gate script on each of your gates, then you can search for the Gate script instead of searching for all gameObjects.

var MyGates : Gate[] = FindObjectsOfType (Gate);
for (i=0;i<MyGates.lenght;i++) {
    MyGates[i].OpenGate ();
}

This will probably work. If you don't have a script attached to the gates you can do a search for all objects with a certain tag, a "gate" tag which you mark all your gates with.

C# example:

Gate[] myGates  = FindObjectsOfType (typeof(Gate));
for (int i=0;i<myGates.Length;i++) {
    myGates[i].OpenGate ();
}

The Stone's answer is correct. also you can use their name to find them. GameObject.Find (string name) is the function that you can use. this function will return null if find no object. the best approach is the code shown above if you have a special script attached to gates. in most cases you have, i think. in this way you can create reusable codes so don't put gate's special code in a general script for gates and doors and windows and ... create a door component and a gate component and ...