The Gunship is the only Helicopter Unit outside mods, and is also the only unit that can go through mountain squares.
The following units can be upgraded to Gunship:
These helicopter units are very fast and quite lethal against armored units. Use them to soften up the enemy, but you will have to use a land-based unit to capture cities.
A gunship is an armed helicopter. They are the cavalry of the modern army, designed to reconnoiter enemy positions, deploy special forces deep behind enemy lines, and so forth. However, modern gunships are armed with armor-piercing missiles and heavy, quick-firing machine guns; with these, a gunship can wreak havoc on enemy tanks and troops.
Gunships first saw extensive service in the Vietnam War. The Soviet Union employed gunships in Afghanistan, and the US has used them to great effect in all its recent conflicts.
Ground troops defend themselves from gunships with SAMs (surface-to-air missiles). When the CIA supplied Afghan guerrillas with such weapons during the Afghanistan war the Afghans used the SAMs with such devastating effect that they virtually crippled the Soviet war effort.