I know this has been discussed briefly in the support forums in the past ( http://en.forums.wordpress.com/topic/gallery-from-media-library?replies=6 ) but I just wanted to echo that WordPress may want to look into creating a solution to this problem. As I understand it, as of now there is no way to create a gallery in a new page containing images from your media library that are uploaded/attached to existing posts. Instead, you have to re-upload, or manually put them in the HTML tab--but if you do that, it still does not show up as a "gallery" in the editor for that page, thus without the gallery options.
There must be some way to start a new page and have the option to create a galley and then just go through and flag the existing pictures in your media library that you want to include. It seems silly to have to re-upload images twice to make a new gallery of them.
The blog I need help with is progressivetimes.wordpress.com.