In readme exists HOWTO:
1. Create your gallery content type or alter an existing one. 2. Add an image/nodereference field to your content type. 3. Go to the Manage Display page of your content type. On your image field in the FORMAT column, choose "Galleria" as formatter in the select box. Next click on the button on the right of the select box and choose your image style presets for the main image ("Zoomed $ image style") and the preview thumbnails. If you downloaded additional themes for galleria you are further able to choose the theme you want to use here. 4. Upload some images, add titles if you want and view the node. You should now be able see the images in the galleria display widget.
But, Galleria formatter is not appears for image field of node, why?