I'm sorry but support doesn't include revert back your installation. You will have to restore your website using a backup.
By the way, php 5 is no longer supported even by php.net. You should really update your server.
I abandoned G\ and most likely I will release a final version when 3.X ends. To be honest, the way that G\ works is very basic and it doesn't suit its purpose anymore (raw speed). Take in consideration that it was created in 2014 so it is very outdated.
The new framework works way better and...
In image navigation is only for images within an album. If you want to browse images with arrow navigation, you can do it directly in listings. You have to use the listing explorer (the full screen image viewer that goes full screen when you click an image when in a listing).