In this sample I managed to add a long press gesture by altering this method in GrowRowTableCell
public GrowRowTableCell (IntPtr handle) : base (handle)
{
    var longPressGesture = new UILongPressGestureRecognizer (LongPressMethod);
    AddGestureRecognizer (longPressGesture);
}
void LongPressMethod (UILongPressGestureRecognizer gestureRecognizer)
{
    if(gestureRecognizer.State == UIGestureRecognizerState.Began)
    {
        Console.Write("LongPress");
        var selectCategory = new UIActionSheet ("ActionSheet", null, "Cancel", "test");
        selectCategory.ShowInView (this);
    }
}
looks like this:
