Update GFlashText.as

there is no need of "-1", just test your code with this:

textField.text = "a";
        if(textField.text.length > 0){
            textField.setTextFormat(new TextFormat(), 0, textField.length - 1);
        }
        textField.text = "_";

you'll get RTE:

[Fault] exception, information=RangeError: Error #2006: The supplied index is out of bounds.
at flash.text::TextField/setTextFormat()