A Juicebox gallery is essentially just a regular HTML <div> on a web page (with the gallery as its content).
By default, a Juicebox gallery will be justified to the left-hand side of the web page that it is embedded into, just like any other <div>).

If your Juicebox gallery has a galleryWidth of 100% and is the only content on its web page, then it will be horizontally centered as there will be nothing to its left or right in the browser window.

If you want to horizontally center a Juicebox gallery within its parent container, then try changing:

<div id="juicebox-container"></div>

... to:

<div id="juicebox-container" style="margin: 0 auto;"></div>