It's almost all or nothing. Without access the user cannot see the gallery or gallery blocks. However, embedded thumbnail images in nodes are still visible (but the link won't work). You also need to set 'mode.embed.only' to true in your Gallery2 config.php file to avoid bypass via standalone. IMPORTANT: You will need to set the user permissions to 'access gallery' for the roles you want to be able to see the gallery.
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