Unfortunately, as you have discovered, the current version of WP-Juicebox (v1.5.1) is not compatible with the new Gutenberg editor within WordPress 5.0 (and later).

We hope to release a version of WP-Juicebox which is compatible with WordPress 5.0 very soon.
In the meantime, there are two things that you can do.

(1) Install and use the Classic Editor plugin (written and maintained by the WordPress team) from here.
This replaces the Gutenberg editor in WordPress 5.0 with the familiar TinyMCE editor and, in doing so, restores WP-Juicebox functionality.

... or:

(2) Try the new version of WP-Juicebox which you can download from here.
It has not yet been released but it has been designed to work with the Gutenberg editor in WordPress 5.0.
The new plugin adds an 'Add Juicebox Gallery' button to the Gutenberg toolbar which, when clicked, displays the regular WP-Juicebox gallery settings window.
After clicking the 'Add Gallery' button, the WP-Juicebox gallery shortcode is entered into the page or post as a Gutenberg 'Shortcode' block.
Here's how to then add images to your WP-Juicebox gallery (essentially attaching images to the page or post) from within Gutenberg:

• Add a Gutenberg 'Gallery' block to the page or post containing the WP-Juicebox gallery shortcode.

• Upload images from your local machine by clicking the 'Media Library' button (in the 'Gallery' block) and dragging and dropping the images into the media window.

• Wait until the images have finished uploading.

• Reorder the images (if necessary) by going to the 'Create Gallery -> Media Library' section, selecting 'Uploaded to this post' and then dragging and dropping the images into the order you require.

• Close the media window using the cross at the top right (do not click 'Create a new gallery').

The new version of the plugin also adds the ability to create a new gallery on the 'WP-Juicebox -> Manage Galleries' page.
This is simply an alternative way to create a new gallery when using an editor which does not support the 'Add Juicebox Gallery' button (such as CodeMirror).
After creating a gallery via this method, you would need to copy and paste the shortcode into the page or post (and then attach images to the page or post as usual).

If you are a Juicebox-Pro user, please remember to upgrade the plugin to Juicebox-Pro (as it comes bundled with Juicebox-Lite) by following the instructions here.

Please remember that the plugin has not yet been released and is still officially in an experimental state (although I have tested it myself and it should work fine).
If you choose to try the new plugin, please let me know how you get on and if I can be of any further assistance.