You can try...

//css

.gritter-center{
   position:fixed;left:33%;right:33%;top:33%}

 

//Javascript

$.gritter.add({ title: 'This is a centered notification', text: 'Just add a "gritter-center" class_name to your $.gritter.add or globally to   $.gritter.options.class_name', class_name: 'gritter-info gritter-center'  });

 

ref: https://stackoverflow.com/questions/18461530/jquery-gritter-set-position