I will try to explain my problem as clearly as possible.
I set up a site with WordPress and got the NextGen gallery working fine.
The gallery is in the sidebar, on all pages, called from the template (not a widget).
The gallery displays 9 images, so there is a pagination.
Then, I set up the site as a multisite.
I have the gallery coming up on 4 different sub-sites (Gallery ID 1 each time).