if your outlet is a UIView and you make the map in code, you have to add it.
Obj-C:
mapView.frame = mySubview.bounds;
[mySubview addSubview:mapView];
Swift:
mapView.frame = mySubview.bounds;
mySubview.addSubview(mapView)
if your OUTLET is a mapview then use that and dont make a new one
mapView = (GMSMapView*)mySubView;