The code you've suggested above will execute on every key up event, which I think would not be what you want, this would effectively stop a user from easily editing the text in the centre of the text box.
If you just want the cursor to start at the end of the text when the text box received focus, perhaps you chould hook into the GotFocus Event, and then use the SendKeys.Send() method to send the End key.
this.tGID.GotFocus += OnFocus;
private void OnFocus(object sender, EventArgs e)
{
SendKeys.Send("{END}");
}