Giving the man a calm smile, the Doctor shook his head gently. "It's possible. If the memory fits so well, how would you know which one had been added?" He could be understanding when the situation required it, and, well, Gabriel's frustration was certainly understandable. The Doctor would be equally frustrated to discover memories that shouldn't be there! "Either they've removed the memory of the ordeal or you've somehow blocked it off due to trauma," he commented. Both were viable.

"Is there anything... any other memories you think are out of place?" Probably a long shot, since the memories would have to come up for him to recognize them for what they were.