It wasn't that Jack didn't believe her. It was that it wasn't making much sense. Owen, yes; Tosh, no way in hell. But with the expression on Gwen's face, Jack knew she was telling the truth. The question remained, however, of when. Of course they were going to die but was it a month into his future or five years?
It was all relative.
Jack started ever so slightly when she grabbed his hand, a look of confusion momentarily passing over his face. Just how far apart were their timelines?