getGMapElementById... mmmhh, sounds good, right?
It works just like it sounds: it gets the Javascript ID of the elements that have been added to the map (normally markers), so that the limits are up to you ;)
It can be used by means of a static method of the GMap class, or as a method of your instance of the control.
Like always, this simple example is enlightening (click on the map and the icon):