If Peter were a healthier person, he might be able to admit that it could be a kind of catharsis for them both, being able to speak with one another - but he's not, so he doesn't. He's not sure he could accept it if it was offered which is why it's so great he's backed into a corner. Still, even he can see that this is affecting her, despite the pokerface she's trying to portray.

This Gwen is not the same. No one ever could be. But he knows, as she said, that she is Gwen.

"You gotta be shittin' me." Peter's tone doesn't suggest that he doesn't believe her, but more that he can't believe this is happening to him. About par for the course, really. He doesn't know why he bothers being surprised anymore. "Transversal theory, black hole cosmology - all, uh, all real?"