Should I close the gable vent on my garage in the winter?
In most cases, vents are there precisely to be open in the winter (and all other seasons) and plugging them up may result in various problems depending on which of various roles they serve, including but not limited to combustion air supply for fuel-burning appliances, keeping the roof from forming ice dams, preventing mold and mildew, etc.

As such, even without further details about what this particular one is doing, I would tend to think that it was put there precisely because there is supposed to be a vent there, usually to meet building code, and it should not be plugged.