Im pretty sure that the gunships are jst sentient beings with a fan and gun grafted into them.
same with stiders, there just enslaved beings just like every other enemy you face, like the alien_slaves and _grunts from HL1. can't you just pictue them with out the weapons walking around there homeworld before the combine enslaved them. and they don't have metal grafted on, its just like a hard shell, like a turtle shell or a labsters exosceleton.
and dropships seem to be of the same race as the gunships, white silvery skin, flexible body. they just have legs and such that have jets grafted into them and its legs/arms are holding up a canister full of troops. which by the way you can put enough rpg rounds in the can to make the dropship drop it and flee.