The alt text and brief description should work fine (I just tested this).
The GP metaboxes (sidebar, disable elements etc..) don’t work.
I researched this, and apparently WordPress doesn’t treat attachments pages like regular post types, so the saving doesn’t fire for these metaboxes.
So, it’s a bug that GP even shows these options on the attachment pages. I’ll make sure they’re removed for the next version.
Let me know about the alt and description though – those should definitely be working.