When you purchase GP Premium, you may assume that it’s a theme which you need to install via the “Themes” area of your WordPress Dashboard. Doing so will result in an error telling you that the theme is missing the style.css stylesheet.
This is happening because GP Premium is actually a plugin.
This means you need to upload it through the “Plugins” area of the Dashboard.
For step by step instructions on how to install GP Premium, check out this article.