what are stackpanel grids for?

You can achieve your goal by configuring GridView.ItemContainerStyle:

<GridView ...>
    <GridView.ItemContainerStyle>
        <Style TargetType="GridViewItem">
            <Setter Property="HorizontalContentAlignment" Value="Stretch" />
            <Setter Property="VerticalContentAlignment" Value="Stretch" />
        </Style>
    </GridView.ItemContainerStyle>
    
    <!-- other code -->
</GridView>