I have an off grade house that was built in 1952. I think it still has only the originaly insulation in the attic. Which is probably only about 2 inches and has two large gabels on each end. I know for sure I need more insulation in the attic, but my question is should I reduce the size of the gabels. Also it is cost effective to insulate the underside of the house? The floor is always very cold.
I wouldn't reduce the size of the gable vents unless your sure about the soffit vents. There is a ratio of inlet/oulet to make sure that the airflows up and out and not down and around to ensure proper air volume exchange. Unfortunately, I'm only aware of such a rule and don't know the specifics. The construction pros should be able to clarify.
I would not restrict the ventilation in any way. Insulation would help considerably, maybe 6 to 8" of blown in celulose. Under the floor could stand 6" of batt insulation held up with spring wires; you could use fiberglass or the new itch free polyester (but I think the polyester will be much more expensive).