var item = new GalleryItem
                        {
                            Key = rep.Id,
                            Text = rep.Name,

                            CommandParameter = report.Id
                            Image = new BitmapImage(new Uri(rep.LargeIcon, UriKind.RelativeOrAbsolute)), 
                            Tag = rep.Description,      

                      
                        };

item has no ToolTipService attribute

Here is how to set a tooltip on GalleryItem.

var item = new GalleryItem
            {
                Key = "newitem",
                Text = "New Item",
                Image = new BitmapImage(new Uri("pack://application:,,,/images/USFlag.gif")),
            };

            // Add the item to GalleryTool
           ToolTipService.SetToolTip(item, "New Item");

Let me know if you have any question.

Sam

I found I just need to replace "New Item" with a new xamRibbonScreenTip instance to achieve my goal.