Shedman has quoted the right sections of the rules.
But if gunships land, they are subject to normal direct and indirect fire and may be assaulted.
I think on-table winged creatures rated as gunships are best treated as Grav troops.
Some gunships have Grav movement (e.g. Andrayadan, Khazari) so if they are used on-table they count as ground troops for fire and assault.
