You actually shared a link to run your app rather than to clone it. (You’re not alone.)
I think I have the idea without seeing your code, though. (Disclaimer: I haven’t actually tried this or used the GoogleMaps integration at all.)
Whenever your code adds a marker to the map, run this method on it (before you lose track of it):
marker.add_event_handler("position_changed", marker_position_changed)
Then you define what happens in response this way:
def marker_position_changed(sender, **event_args):
marker = sender
print(marker.position) # or store to your database, etc.