Moving the Pola gates would be troublesome if you were doing it more than once each year. That's because they must be wired to LGB 17100 track switches. Those are the little black boxes that fit between the rails and are used to activate various items like switch tracks and signals.
You could use quick connect plugs to make removal of the gates simpler, but I don't recommend it on a daily, weekly or even monthly basis.
The best thing you can do is fabricate a cover for the winter. A simple plastic storage container would serve the purpose well.
I don't know about the LGB crossing gates being left outdoors indefinitely, however, all LGB equipment is meant for the outdoor environment, so my guess is that it would be fine.
I believe the LGB gates work by the weight of the train passing over the boards you mentioned. The only thing about this arrangement would be keeping the mechanism clear of debris, like seeds, twigs, leaves and so forth. I have never owned a set of these gates so I don't know how much maintenance is involved.
You have my interest now. Perhaps this summer I will try a set of the LGB gates myself.