You’re trying to install the free theme as a plugin.
To install GP Premium, you must first download it through your account (https://generatepress.com/account) or through your receipt email.
The file will save to your computer as gp-premium.zip – then you can upload it as a plugin.
More info here: http://generatepress.com/knowledgebase/install-activate-addons/
Let me know if you need more info 🙂