Unit specialized in fighting Modern Armor and Tanks. It is capable of hovering over mountains.
The Helicopter Gunship is a deadly air unit that in most respects acts like any ground unit. Its movement of 6 makes it extremely quick, and it can move over all terrain types including mountains as if they were clear, but it must embark to cross water. Gunships receive a hefty combat bonus when fighting tanks.
Helicopters saw limited use as transport vehicles in World War II and the Korean war. During the Vietnam war the US Army decided to create a more heavily armed and armored chopper designed specifically for combat. Since that period, the helicopter gunship has evolved primarily into a tank-killing machine. Able to fly extremely close to the ground, "pop up" for a quick shot and then scoot away at low levels before the enemy armor could respond, helicopters were almost unstoppable except by air power until the creation of handheld "suitcase" surface-to-air missiles like the ones the Mujahidin used against the Soviet choppers in Afghanistan. Modern combat helicopters perform other functions, like close air support for infantry units or rapid transport for special ops teams, but their primary mission remains killing tanks.
The Helicopter Gunship is modelled after the Russian Mi-24 Hind, one of the most heavily armed helicopters in the world.
While the helicopter gunship is primarily to take down enemy tanks, it also makes a great first-response defender, being able to reach an invading enemy regardless of difficult terrain. An armored blitzkrieg can be stopped cold by a small amount of helicopter gunships, giving time for the rest of one's army to react. It also excels in it's own blitzkriegs, it's speed meaning it can take down key targets such as artillery and armor so your own army can storm in relatively unhindered. It's never a bad idea to keep a few of these around, though be wary of Mobile SAM and aircraft, as they can deal massive damage if you let them close in on your gunships.