Usual disclaimers apply

Tosh was the sacrifice Jack had to make because he was too selfish to let her leave after he had saved her from UNIT.

Owen was the sacrifice he had to make because of his own brother.

Gray was the sacrifice he had to make because he let go of his hand all those years ago.

Ianto was the sacrifice that Jack had to give because he didn't think about what the 456 would do to them if they fought.

Steven was the sacrifice that Jack had to give to save the world.

Alice was the sacrifice he had to give for killing Steven, for saving the world.

John was the sacrifice he had to make because of his rage at Ianto's death. He killed John Hart because he lost control.

And Gwen…leaving Gwen was no sacrifice at all. It was relief.