I don't recall a tattoo ever coming up for a mage... As someone stated already, you receive a ring with the circle insignia (not the circle with the line through it, but the one that is sectioned into four parts each with a symbol for the "approved" taught schools of magic).
Also, being a Harrowed mage marks you somehow. It's the reason why Jowan needs your mage to get into the Phylactery chamber, only a Harrowed mage can help open the door. But I thought Anders had at least gone through his Harrowing
EDIT: Also I don't see how Gatt would have been able to tell if you were Harrowed, since he wasn't a mage or Templar that could tell. So I can only assume "the mark of the circle" can only refer to the ring, or some kind of insignia you wear.