I come here with the same doubt but for Xamarin for Android, I have used the Sigrist answer to do this method after save my file:
private void UpdateGallery()
{
Intent mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
Java.IO.File file = new Java.IO.File(_path);
Android.Net.Uri contentUri = Android.Net.Uri.FromFile(file);
mediaScanIntent.SetData(contentUri);
Application.Context.SendBroadcast(mediaScanIntent);
}
and it solved my problem, Thx Sigrist. I put it here becouse i did not found an answare about this for Xamarin and i hope it can help other people.