1) Gallery Forums are intended to all be related to the main Gallery Post.
These are intended to be like a physical gallery. One where you go to this destination and see the works that an artist has created. You can go in and enjoy the works, but you cannot bring your own items and set them up next to the OP's work -- you need to make your own Gallery Topic to display your own gallery.
Along those lines I can see where it might be useful if the OP could add additional items, perhaps as attachments
Right now a new reply to a Gallery thread shows up in 'Active Threads' and 'Active Posts', but it can't show up in a 'Post Island' because Gallery is not included in the list of Forums to choose from.