Neither of the versions can be downloaded using the link
Topic on Extension talk:HoverGallery
Are you sure? It works for me.
The link's error is because of a capital letter G (HoverGallery) returns an error.
Kghbin has it correct in the above link with a path leading to 'Hovergallery'
The repo has been renamed to HoverGallery (with capital G) so this should no longer be an issue.