Gallery image must open up in fancybox directly not post page

Make sure that your galleries are set as Links to “Image File” and not to “Attachment Post” for Galleries and images. You’ll find the settings in the image/galleries pop-up window.

I’m not familiar with your plug-in but you might need to manually add the fancybox class to each image. Another option is to add the code to your theme manually and skip the plug-in. I’ve posted an answer here: Any good lightbox-like option that works?.