How to show a Info window in iOS Google maps without tapping on Marker?

This has changed on Google Maps SDK and it's easier to understand:

GMSMarker *marker = [[GMSMarker alloc] init];
marker.position = coordinate;
marker.title = @"Location selected";
marker.snippet = @"Testing";
marker.map = mapView_;

//Show info window on map
[mapView_ setSelectedMarker:marker];

You use now setSelectedMarker method to show an info window of a marker

GMSMarkerOptions *myLocationOptions = [GMSMarkerOptions options];
myLocationOptions.title = @"My Location";
myLocationOptions.snippet = @"Lat:...., Lang:....";

mapView.selectedMarker = [mapView addMarkerWithOptions:myLocationOptions];

(note that it's Options, not Option)

Swift 3.0

func addMarker(_ location:CLLocation){
        var locationMarker: GMSMarker!
        if locationMarker != nil {
            locationMarker.map = nil
        }
        locationMarker = GMSMarker(position: location.coordinate)
        locationMarker.map = mapView
        locationMarker.appearAnimation = kGMSMarkerAnimationPop
        locationMarker.icon = GMSMarker.markerImage(with: UIColor.green)
        locationMarker.opacity = 0.85
        locationMarker.isFlat = true
        locationMarker.snippet = "My Location"
        mapView.selectedMarker=locationMarker

    }

below line is the answer

mapView.selectedMarker=locationMarker