Lead Developer
You can re-install by:
1. Activating another theme
2. Deleting GeneratePress
3. Re-installing GeneratePress: https://generatepress.com/knowledgebase/installing-generatepress/
That will make sure you have all of the default files.
Let me know 🙂
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development