Full code below (gmapExtras is a object of type GoogleMapExtras)
B4X:
Dim MarkerOptions1 As MarkerOptions
MarkerOptions1.Initialize
MarkerOptions1.Position2(lat, lng).Visible(True)
Dim BitmapDescriptor1 As BitmapDescriptor
Dim BitmapDescriptorFactory1 As BitmapDescriptorFactory
BitmapDescriptor1=BitmapDescriptorFactory1.FromAsset("vor.png")
MarkerOptions1.Icon(BitmapDescriptor1)
MarkerOptions1.Rotate(90)
Dim Marker1 As Marker = gmapExtras.AddMarker(gmap, MarkerOptions1)