I believe what you want is to add the delegate and override the didTapMarker
method
/**
* Called after a marker has been tapped.
*
* @param mapView The map view that was pressed.
* @param marker The marker that was pressed.
* @return YES if this delegate handled the tap event, which prevents the map
* from performing its default selection behavior, and NO if the map
* should continue with its default selection behavior.
*/
- (BOOL)mapView:(GMSMapView *)mapView didTapMarker:(id<GMSMarker>)marker;