I opened the Hammer editor and added a Gunship to get the size of its bounding box, which should give us an approximation of its nose-to-tail length:
These are in "Hammer Units". According to the Valve Developer Community, there are 16 Hammer Units in a foot for architecture, but 12 per foot for human character models. This gives the length as something less than 45 feet.
To be a little more specific, I rotated it and got as close to the length you wanted (I didn't include the barrel on the front, but I did include the tail):
530 units gives us a length between 33 feet (for 16 units per foot) and 44 feet (for 12 units per foot).