As noted in the short description for galleryDescription on the Config Options page, this options text is:

If you enter a Gallery Description, then it will be displayed on the gallery's Splash Page, displayed by default for embedded galleries on mobile devices or when setting showSplashPage="ALWAYS" (in JuiceboxBuilder-Pro's 'Customize -> Splash Page' section).

The Gallery Description will also be used in an Open Graph 'description' tag on the gallery's 'index.html' page (used, for example, by Facebook), e.g.:

<meta property="og:description" content="This is the Gallery Description." />

If you set addSEOContent="TRUE" ('Customize -> Sharing'), then the Gallery Description will also be included in the SEO content code in the gallery's 'index.html' file, e.g.:

<!-- Image gallery content for non-javascript devices -->
<noscript>
    <h1>Gallery Title</h1>
    <p>This is the Gallery Description</p>
    <p><img src="images/IMG_0001.jpg" title="Image Title" alt="Image Caption" /><br>Image Title Image Caption</p>
</noscript>

More information about SEO content within Juicebox-Pro can be found here.
If you'd like text to appear in the gallery itself, then please use the Gallery Title ('Customize -> Lite') or the images titles or captions.

I hope this helps to clarify things.