I rarely use GTC because I am in front of the computer all day. I had a broker friend who told me years ago when I first started trading in the mid 90's that GTC orders would get passed over a DAY order. I have never ever seen this written anywhere so I have no proof that it is true but it always sticks in the back of my head. I would rather set a new day order each day. As far as your broker not allowing GTC, you need to call them and question this. That should not be.