If there is no players in your team that has a RPG equiped, then the only way to take down a Gunship (if on the attacking side in order to push up) is to attempt to shoot it down by regular bullets. Since there is little option for more than couple of players to play Gunner, there should be a weak spot for all other classes to hit for critical damage.
I suggest that a small weak spot on the Gunship itself should be included for the Insurgent team to try to shoot at, like the tail rotor for example so the Gunship will loose control and balance. I don't know where a soldier should aim at on an enemy helicopter in a realistic situation, but there should be a vulnerable shot on the helicopter itself other than the underside. If any does know, or clarify vulnerable points of a helicopter's anatomy, please comment about it!