alinamn
Hi Alina,
I recommend you to use package for J!3, Joomla 2.5 is out of date and not be supported by Joomla or by us anymore.
GK events quickstart J!3 = This is quickstart package of template, you will install it like the way you install Joomla package but it will include sample data so that your site will look like our demo site.
GK events J!3 = This is template package, if you don't want to use sample data, you will install Joomla first (or if you already have an existing site with Joomla installed), you just need to install this template package via Back-end > Extensions > Install
GK events rest files J!3 = This package contains files for plugins used in template, if you use Quickstart package, you don't need this package, if you install template package separately, you download this package and install plugins inside.
I hope this makes sense.
Regards