Hi there,
generatepress.1.2.4 is the free theme file – that’s not the file you want to install as a plugin.
When you purchased, you should have been emailed a download link for GP Premium. If you made an account, which it looks like you did, you can go to your account page (https://genertepress.com/account) and download the file as well.
Then install that .zip as a plugin, and you should be good to go.
More info here: http://generatepress.com/knowledgebase/install-activate-addons/
Let me know if you have any other questions 🙂