I wouldn't call it "straightforward" but it is definitely computable. I worked on such a cartography-video overlay system about 10+ years ago, and you need at a minimum: precise GPS (as you pointed out), distance offset between GPS antenna and camera gimbal, precise aircraft attitude in all three axes, precise gimbal attitude, and a very good terrain model giving you altitudes of the land around the camera target. You also need to project your location, camera vector, and field of view polygon into a coordinate system that takes into account the curvature of the earth, because these aircraft are often many km from their targets. Very interesting project.