I think Gwen did what was best in the circumstances. She couldn't look like a weak queen in front of the whole court by letting the girl off for treason - it was a pretty serious crime. She also will have felt angry and betrayed by her servant as she thought they had some kind of bond but it seems to Gwen that she was just using her for information and, most of all, in Gwen's eyes Sefa could well have got Arthur, the person she loves most in the world, killed. She just contained her anger and did her best to look composed in front of all the onlookers so as not to look out of control. I think she was justified in her actions, though I do think she'll probably let her off in the next episode (just realised that's tonight - yay!)
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