Because you have "page link to: not linked" the NextGEN plugin is generating the gallery page for you and thats a page you cannot edit.
To add text to a galley page it must be a 'real' page ie. it must be in the list of pages that appear under the WordPress Pages tab.
I create my galleries on 'real' pages by selecting a suitable parent page from the "Create new page" dropdown list and clicking on "Add Page". That instructs NextGEN to create a child page under the parent page with the name of my gallery and writes onto the page the correct shortcode for that gallery.
The new gallery page will then appear under the WordPress Pages tab which means I can now edit it like any other WordPress page to add text, banners etc.
!!!Presumably your site is live so you should exercise caution trying this out!!!
Before changing any existing galleries maybe try creating a test gallery to get a feel for how it works. It looks like project-gallery is a real page (?), if so, maybe try creating a new gallery 'test' following the above method and copy some photos from another gallery into it. Then edit the Page 'test' to add some text - as thats your goal. Add 'test' gallery (drag/drop in Album tab) to the ablum that's on the project-galley page. Then check 'test' appears on the project-galley page and when the 'test' gallery thumbnail is clicked it shows the files you copied in and the text you added.