Why surprised that the attackers have the advantage on Gravelpit? That's why I hate the damn thing - it's very difficult to defend, offers little to the defender. And that's why it's most peoples favourite - because it favours the attack-minded majority.
I think I disagree a bit about Gravelpit being an attacker friendly map. With equal teams its very balanced map - Cap B being the major difficulty at first, then C a harder prospect with a determined defence.
I've seen many wins for Blue and Red on gravelpit and think its a more even prospect that say, Goldrush.