If you set imageVAlign="CENTER" rather than imageVAlign="TOP", then the gap will be reduced (the main image will be vertically centered in the gallery's image area rather than aligned to the top, which leaves a noticeable gap at the bottom).
I have tried this and it does look better, but what I really want to do is raise the text to avoid so much scrolling downward on the homepage.
Your gallery also uses the default value for imageScaleMode ('SCALE_DOWN') so images smaller than the gallery's image area are not scaled up to fill it. Try either feeding your Juicebox gallery larger images (so that they are always large enough to have to be scaled down and fill the image area either left-to-right or top-to-bottom, depending on their aspect ratio) or set imageScaleMode="SCALE".
Some time ago I had the site set-up as you are suggesting, but there were some problems I was encountering and I changed it to the current settings per your suggestion. I don't remember exactly what the original problem was.
Another solution might be to reduce the height of your Juicebox gallery in 'Site -> Customize Theme -> Gallery Pages -> External Fit px'. Enter a negative value to reduce the height of the gallery.
Is it possible to do this without affecting other pages of the site? The homepage is the only page with this issue.