Against rush mentality Alliance, it only takes 15-20 Horde to force a stalemate at Galv. I've had this happen several times when I've been part of a rush, and no mater what I said in raid chat, the Alliance still continually threw themselves at Galv in groups of 5 or 10. Eventually, they got him, but by this point the horde had taken every bunker and were already working on Van.
Honestly I don't know why the Horde don't do it more often. Horde defending leads to some real PvP, and if they play it right they'll win. not quickly mind you, but when Alliance players would rather leave instead of fighting, the trickle of players leaving the front lines and replacements having to ride down will allow the horde to fight with greater numbers.