What are the names of the 12 giants that were sent to replace the gods.

Actually, there were much more than 12 giants in the Gigantes war, and not every god had one. I believe Aphrodite was missing one, and Demeter too... And not only the Olympians had them; Hecate, Chiron and the fates fought as well.

Here's a complete list from a site that RR recommends on his website (meaning its accurate and that he himself might use it!) I doubt we'll be seeing all of the giants or that they won't switch around who fights with who, but here you go (scroll down to the bottom);

link

Go to camp half-blood wiki and search for it.