As
Andrew R Stewart said find replacements first. Because Giant loves their proprietary seatpost designs, make sure you can find new parts that will fit and either make sure they have good supply or buy them right then and there. However you will probably be at a Giant Dealer so you might consider chatting with them and seeing if they can help.
If you can get movement out of the seatpost I would try moving it up and out, maybe some good whacks with a mallet on the under side of a junk saddle with the bike upside down (IN A STAND) a little extra gravity might help (you might also try a couple whacks the other way on the top as well just make sure you can loosen any bonds you might have made. If you had movement I would say try and turn it a bunch to get out but Giant put a kibosh on that method but whacking top and bottom might help loosen it. If the bolt was stuck I would say use an impact gun and pull it right out. We had a stuck bolt and the mechanics kept breaking t25s so I said let's try the impact and boom it was out in a second with no broken tool. It needed the quick power that our weak human arms couldn't provide.