There is no way to do this with current version of the SDK (1.4.3), and actually there is an open issue with this request: Look here.
As a work around, you can hide the default button with:
 _map.myLocationEnabled = NO;
Then create a custom GMSMarker
 GMSMarker *pointMarker = [GMSMarker markerWithPosition:currentPosition];
 pointMarker.icon = [UIImage imageNamed:@"YourImage"];
 pointMarker.map = _map;
And change the position of it using a CLLocationManager, so it always show the current position. It's a bit tricky but is the only way, I could think, that you can achieve this. If you need a more complete example let me know.