The GPS and Map item usage is messed up for vehicles in general. The MH/AH-9 helicopters don't require this item to be present in your inventory (or in the vehicle inventory) to use the Right Ctrl+M GPS window or map (M). There are no MFD screens inside the helicopter cockpit - thus you should need a portable GPS/Map item for this window to function at all. The same issue is also with the map. If you don't have a map item on you - you can still use map screen in MH/AH-9.
Now compare this with let's say the Offroad vehicle. The offroad vehicle has an LCD screen and I bet they have a GPS/Map built-in by 2030s. Yet without the portable GPS/Map items - you can't use the GPS nor the Map inside this vehicle.