Clear the cache
on your DokuWiki server.
If you can't get into your Admin screen, you will have to do this via the server backend, but if you managed to update your Gallery plugin without your site working, then I guess you have access to the server back end.
As suggested in the link above, edit local.protected.php and save it using Linux command line touch command, or a Windows text editor, or even using your web hosting GUI if that's relevant.
What kind of server are you hosting it on? Your own/webhosting? Windows/Linux?
Once you have cleared the cache on the server side, try using Private Browsing mode in your web browser to ensure that you are not caching on your PC.
You're not the first person to have this issue
, and others have solved it
, so don't give up :-)
Finally: could you please post the full contents of lib/plugins/gallery/plugin.info.txt from your server?