I used /summon Giant. The giant appeared, but it won't attack or move on it's own. It can be killed and moved forcibly, but nothing else. I was on adventure mode and then survival mode. It still won't attack.
They don't do anything because Giants themselves are unused. Since 1.8, they do not have any AI at all. They're generally unused now and serve no purpose other than decoration.
From the Minecraft Wiki entry:
Giants currently have no AI. The consequence of this is that they always face south when summoned and stay that way, never wandering, turning their head, chasing the player, or responding to damage except for taking knockback. Their viewing direction can only be changed with commands.