The gunner modes are as follows:
Direct gunner mode is the default, you control the gun manually
Land mode (pressing W) works in conjunction with the clipboard map. You bring up the clipboard map, hit W, and then click anywhere on land within gun range and your gun will be automatically given the correct bearing to hit that spot. Remember, you can zoom in to a great degree on the clipboard map to get a very precise firing solution. Once you have clicked the spot on land you want to hit, fire away.
Sea mode I do not use but I am assuming it is the same thing, only with the water on the clipboard map being clickable instead.