After reading the 1 star reviews i decided to install this plugin on a text database firs. Big mistake. Immediately after activating the plugin i got a 404 error and was not able to access the site.
If you do proceed you can fix this quite easily.
The plugin creates a file called 'GZipNinjaSpeed_install_backup1467737212.htaccess' in the root Wordpress folder. All you need to do is delete the current '.htaccess' file from the root folder and then rename the 'GZipNinjaSpeed_install_backup1467737212.htaccess' to 'htaccess'
This restored the access to my site.
Developers of the plugin you need to sort this our or make people aware there is a risk of messing thing up and give them instructions on how to resolve it.