MainPage.xaml

<Page x:Class="ShadowBorder.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="using:ShadowBorder"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Page.DataContext>
        <local:MainViewModel x:Name="ViewModel" />
    </Page.DataContext>

    <Grid>
        <GridView ItemsSource="{Binding People}"
                  ContainerContentChanging="GridView_ContainerContentChanging"
                  Margin="5">
            <GridView.ItemTemplate>
                <DataTemplate>
                    <Grid>
                        <Canvas HorizontalAlignment="Stretch"
                                Margin="10" />
                        <StackPanel Background="DarkCyan"
                                    Padding="5"
                                    MinWidth="150">
                            <TextBlock Text="{Binding Name}"
                                       FontWeight="Bold"
                                       Foreground="PapayaWhip" />
                            <TextBlock Text="{Binding Age}"
                                       Foreground="FloralWhite" />
                        </StackPanel>
                    </Grid>
                </DataTemplate>
            </GridView.ItemTemplate>
            <GridView.ItemContainerStyle>
                <Style TargetType="GridViewItem">
                    <Setter Property="HorizontalAlignment"
                            Value="Stretch" />
                    <Setter Property="HorizontalContentAlignment"
                            Value="Stretch" />
                    <Setter Property="VerticalAlignment"
                            Value="Stretch" />
                    <Setter Property="VerticalContentAlignment"
                            Value="Stretch" />
                    <Setter Property="Margin"
                            Value="5" />
                </Style>
            </GridView.ItemContainerStyle>
        </GridView>
    </Grid>
</Page>

MainPage.xaml.cs

using System.Numerics;
using Windows.UI;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Hosting;

namespace ShadowBorder
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private void GridView_ContainerContentChanging(ListViewBase sender, ContainerContentChangingEventArgs args)
        {
            if (args.ItemContainer is GridViewItem gvItem && gvItem.ContentTemplateRoot is Grid rootGrid)
            {
                // Get the elements in the DataTemplate using index (this isn't very safe but you get the point)
                var canvas = rootGrid.Children[0] as Canvas;
                var stackPanel = rootGrid.Children[1] as StackPanel;
                var visual = ElementCompositionPreview.GetElementVisual(stackPanel);

                var sprite = visual.Compositor.CreateSpriteVisual();
                sprite.Size = visual.Size;
                sprite.CenterPoint = visual.CenterPoint;

                var shadow = visual.Compositor.CreateDropShadow();

                // Change this to make the shadow bigger or smaller
                shadow.BlurRadius = 10;

                // Change this starting position if you want the shadow a little higher or lower
                shadow.Offset = new Vector3(0, 0, 0);

                // Darken this color if you need darker shadows (I tested against a white background)
                shadow.Color = Color.FromArgb(0x96, 0x96, 0x96, 0x96); 

                // We're done setting up the shadow, set it to the sprite visual
                sprite.Shadow = shadow;
                ElementCompositionPreview.SetElementChildVisual(canvas, sprite);

                // animated the final result using reference parameters
                var bindSizeAnimation = visual.Compositor.CreateExpressionAnimation("hostVisual.Size");
                bindSizeAnimation.SetReferenceParameter("hostVisual", visual);
                sprite.StartAnimation("Size", bindSizeAnimation);
            }
        }
    }
}

MainViewModel.cs

using CommonHelpers.Common;
using CommonHelpers.Models;
using CommonHelpers.Services;
using System.Collections.ObjectModel;

// IMPORTANT: Add "CommonHelpers" NuGet package for sample data source https://www.nuget.org/packages/CommonHelpers/
namespace ShadowBorder
{
    public class MainViewModel : ViewModelBase
    {
        public MainViewModel()
        {
            People = new ObservableCollection<Person>(new SampleDataService().GeneratePeopleData(true));
        }

        public ObservableCollection<Person> People { get; set; } = new ObservableCollection<Person>();
    }
}