Turns out that some settings from drupal’s .htaccess file were filtering down into the gallery directory. I had
display_errors set to off so I couldn’t see what was wrong.
Once I’d turned on errors and saw:
"Fatal error: Failed to initialize session module in /some/local/directory on line 51"
I had one of those eureka moments.
So, for anyone else running drupal and gallery together, if you have a similar problem, then just place this entry in the .htaccess file to set session handling to something more to Gallery’s liking.
php_value session.save_handler files
Now I just need to tweak to styles, and get a link into the main navigation and we’re good to go.