I realise that you do not want to use Flickr but, as Juicebox has built-in support for Flickr, using a Flickr account (or a Flickr set) as a source of images for a Juicebox gallery is a very simple process of just setting one or two configuration options.
Unfortunately, Juicebox does not have built-in support for Google Photos and there is no quick and easy solution for displaying images from a Google Photos album in a Juicebox gallery.
It is certainly possible to use a custom data source (as noted in this FAQ: Can Juicebox handle a custom data source, for example RSS or Instagram?) but Google Photos integration would likely turn into quite a substantial coding project.
You'd probably need to use the Google Photos Library API with the PHP client library (which looks to be quite a complex process in itself) to obtain a list of photos and then dynamically create the XML data for your Juicebox gallery in the correct format (using PHP).
I do not know of anyone who has done this before for Google Photos so I am unable to point you in the direction of any sample code.
Depending on the scale of your project, it might be quicker (and certainly easier) to just duplicate your Google Photos albums for your Juicebox galleries (using local images).