Board Thread:Questions and Answers/@comment-5729782-20121111105405/@comment-5286206-20121117150249

'Merlin knows he must act fast before Morgana’s twisted control over Gwen gets further out of hand.

But only the most powerful magic can save the Queen and this will not be easy, even for Merlin. As the High Priestess tightens her grip on Camelot, one thing is clear – she will not give up her puppet without a fight'

Source: http://cultfix.co.uk/merlin-series-5-episode-9-synopsis-19747.htm

Here you go, so Gwen will have to have powerful magic imbued on to her.