in reality, it would be a c-130 hercules, since the ac-130 spectre does not have any room for cargo. considering the laziness of iw, they used the same graphic for the plane.
anyway, the usaf first used gunships based off of cargo planes during the viet nam conflict. the first converted cargo plane was based upon the c-47 (civilian version was the dc-3) first used back in world war 2, the ac-47 spooky. next was the ac-119 shadow/stinger, followed by the ac-130.