P1: 676 Million.
P2: 1757 Million. (only goes to 10%, then P 3 starts)
P3: Heals for 30% health (that being 3x 175.7 = 527.1 Million)
P4 (Heroic only phase): Heals back to 60%, that being 1054.2 Milllion.
So, if we add up all of his health, Garrosh would have
676 + 1757 + 527 + 1054 = 4014 Million HP (a bit more actually, but yeah).
And I got those values from Method's 25 Man Heroic World first Video.