Thanks! Very helpful solution.
The following example is the solution using tag.
def marker_click(sender, **event_args):
i =GoogleMap.InfoWindow(content=Label(text=sender.tag))
i.open(map, sender)
marker = GoogleMap.Marker(position=position)
marker.tag = 'the text you want to show'
marker.add_event_handler("click", marker_click)