How can I create the event that will raise when I select an item from the gallery?
You can use the GalleryItemMouseLeftButtonDown event of the GalleryGroup to handle item selection in the GalleryControl.
[XAML]
<syncfusion:GalleryGroup GalleryItemMouseLeftButtonDown="GalleryItemMouseLeftButtonDown_Click"/>[C#]
//The code to handle the above event is
private void GalleryItemMouseLeftButtonDown_Click(object sender, MouseButtonEventArgs e)
{
// Your item selection logic here
}