He didn't have anything to say for a very, very long moment. It was clear, despite Gabriel's attempt to conceal it, that he was upset at the loss of the Doctor. Even though it had become clear that people could simply disappear like that, Gabriel had clearly never expected it to happen to someone he was close to, as opposed to a potential threat.

Eventually, he squeezed Gabriel's arm in a poor attempt at comfort and let go. "Well, we'll simply have to find out for ourselves, then," he said as calmly as he could, trying to ignore the panic that was beginning to edge into his consciousness.