I notice you use a galleryHeight of 50%.
This suggests that the gallery's actual height should be only 50% of the height of its parent container (and this would likely result in unwanted space).
Also, when using a percentage height, you'll need to make sure that all the gallery's parent containers have heights assigned via CSS, otherwise the browser may not be able to determine what the gallery's actual height should be. (The browser needs to know what the gallery's height should be 50% of.)
Additionally, you might need to take into account this note regarding Using Percentage Heights.
You might like to try setting your galleryHeight to 100% (so that you know that the gallery will always fill the height of its parent container) and then adjust the height of your gallery's parent container(s) as necessary.
Otherwise, to avoid the potential problems associated with using a percentage height, I'd recommend setting your galleryHeight to a fixed pixel value (such as 600px).
In doing so, you will have total control over the height of your gallery (you always know how high it will be), and being that your web page has a lot of vertical scrolling content, your gallery would not benefit from being responsive in the vertical dimension.
At the moment, the height of your gallery (and the amount of space above and below the main images) changes depending on the height of the browser window. I expect you probably do not want this to happen and a fixed height gallery would be the solution. (A vertically responsive gallery would really only be useful if your web page's content filled the browser window without any vertical scrollbars.)
Try using something like the following (adjusting it if necessary) and hopefully you'll be able to reduce the space surrounding your images.