There had been no warning and at the same time, Jack never made a move to try and stop Logan either. He figured Gambit had it coming, no matter what happened. No warning shout, no nothing. He simply stayed where he was, crouched and ready to move the next piece of wood with the Doctor.
"Now, that's what I call getting nailed." Really, he couldn't help himself. The backflip, however, was rather impressive, a note Jack filed away in the back of his head. Gambit certainly wasn't what he appeared to be.
None of them were when it came right down to it. So was that why they had been collected? To be studied and watched for what extra abilities they had? It was quite the chilling thought.