From the research I have done it looks like the book and the movie are a little different... shocker. Using the details from the answer for this question here the gate is actually three stages: The planetary shield, gate housing, and an inner shield for the gate itself. This makes a lot of sense.
A statement made by Rogue One co-producer John Swartz here makes the idea that the gate in the film is just two stages: One massive shield that encompassed the entire planet and then a "gate barrier" that moved two massive wedges into and out of position. The two wedges that were used to hold the gate open only work in one lateral direction, otherwise it would have been a giant ring that was raised and lowered to open and close the gate. (Though that does not really apply to your question I thought that was interesting from watching the movie...)
So we can all see the giant wedges that are being removed from blocking the shield to allow it close. They are slow moving and in contrast to the co-producer's point "punching" through puts the image of more speed and force in my mind. Therefore, in my mind I see those gates that hold the shield open more like mirrors than dams.
Given that a craft can not simply punch through at a high enough speed there is something else the Empire has involved that would allow the wedges to move into position. Lowering the entire planet shield to "re-arm" would almost defeat the purpose as well as the name gate; and from the comment of "punching through" they do not lower the shield. I am still looking to see if there is any mention of a material or coating for the wedges of the gate that makes them special, but i do not think that the movie was worried about this.
I was hoping this article would have more details, but it does not. Still very interesting to see the concept art and some of the earlier ideas.