How do I add "Content" to a <GroupBox> that contains a <Label>?

Question

Snippet of xaml code:

<GroupBox VerticalAlignment="Top" Height="58" Margin="0,0,0,0">
    <StackPanel Orientation="Vertical">
        <Label Content="Players" 
                    Margin="0,0,0,0"
                    HorizontalAlignment="Center" />
        <TextBox x:Name="gTBx" 
                        Margin="0,-2,0,0" 
                        Width="30"
                        HorizontalAlignment="Center" />
    </StackPanel>
</GroupBox>

I see snipboard.io/WAQwDB.jpg

It doesn't like it when I add "Content" to the <GroupBox>    =>    snipboard.io/AY4IqU.jpg

<GroupBox VerticalAlignment="Top" Height="58" Margin="0,0,0,0" Content="Number">