How do I have the first keyboard entries start in a specific TextBox?

You can do as below,

using System.Windows;

namespace WpfApp
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void StackPanel_Loaded(object sender, RoutedEventArgs e)
        {
            gTBxNumberOfPlayers.Focus();
        }
    }
}