User blog comment:FlamingShadow213/Who do you think will find out in season 5?/@comment-4892425-20121003112552

If Merlin reveals his magic to Gwen I don't think she will take it very well. One, the lying for all these years. Two, Merlin could have used his magic to clear Gwen of adultery with Lancelot and she could take his lack of action as fear of exposure. Three, now that she's queen it puts her in a tough position to keep this from Arthur; Merlin would be asking her to put her marriage on the line. When the hints are coming out that Gwen's and Merlin's friendship is tested this could be why.