I see a couple of possibilities for a gate. One would be something custom-fabricated by a shop that makes porch railings, for example. Another would be something like a frame with heavy woven wire screening in the center. Either type could be attached to brackets screwed to the door posts outside the door itself, and could be left in place but easily lifted off the brackets as necessary.
Inside the car, to protect the door when open, a "crate" could be built using 1 x 4 or 1 x 6 vertical boards from floor to ceiling, spaced two or three inches apart to allow some light from the window to enter the car.