A small update after a long time: After my web host SDF.org changed the HTTP server from good old Apache to Nginx last year, the Gallery has been broken, mostly due to lack of support for user-specified Apache
mod_rewrite rules in Nginx. Fortunately I’ve now gotten it sorted out with a little assistance from the SDF admins and the very helpful people at the Gallery3 mailing group. Some server configuration for PHP had to be changed, and it was actually just a pretty small fix.
The Gallery Project has been around since 2001, when I first installed it. Unfortunately the project’s been in hibernation since 2014, but lately there’s been an attempt to make it work on newer PHP versions and fix some bugs, called Gallery Revival, and the code is available on GitHub.