Using GUI Button to activate script instances

You cannot access the function spawner because your script doesn't know where to get it from.

Also GetComponent gets a component attached to the specified gameobject (in this case the script to which your GUI script is attached) of the type specified. A function is not a type of component.

If your function were static, you could access the function spawn.spawner (static function spawner in the script spawn.js), but the variable transform would be invalid to use within the static function because it is not static (as would spawnObject, but depending on your use case is less problematic).

Because you want to call the function on instances of the script, you will need to get the instance(s) of the script attached to your other GameObjects like so:

button.js

function OnGUI () {
    if (GUI.Button (Rect (10,10,150,100), "Drop")) {
        //Find the spawners. Requires they be tagged Spawner
        //If these don't change, you could store this on Start
        //or you could store them publicly (and even statically if you like).
        var spawners : GameObject[] =
            GameObject.FindGameObjectsWithTag("Spawners");
        for(var spawner : GameObject in spawners) {
            var script : spawn = spawner.GetComponent(spawn);
            if(script) //If they have this script attached call the function
                spawn.spawner(); 
        }
    }
}

As an alternative, depending on your use case, in stead of having the spawner function in a script attached to your spawner GameObjects, you could store the GameObjects and the function within the button or another controller script that is easier to access (maybe as a static function?) and then have the spawner function loop through the GameObjects that are stored with it and instantiate, like so:

spawnTarget.js (Attached to your spawners)

var spawnObject : Rigidbody;

spawn.js (Doesn't really need to be attached to anything, but it can be)

static var spawners GameObject[];

button.js (Attached to something)

function OnGUI () {
    if (GUI.Button (Rect (10,10,150,100), "Drop")) spawn.spawner();
}