Gwen's gullibility is one of her biggest problems. She'll believe anything except the truth, or anything she wants to believe badly enough. There's no way this Stryker person is UNIT, she's bringing danger down on Jack's head, and Ianto's too. If she had half a brain she would have checked Stryker's credentials with UNIT, but she just wanted an excuse to go running after Jack and tell him off for leaving her and disbanding Torchwood. That's all very true to character. I feel bad for Rhys.