It should work with System fonts, but hosting your fonts locally in your website won’t mean that it will become a system font. It’ll just mean that, “hosting fonts locally in your website”.
As for the credentials, I believe the username is missing?
Can you double check if you’ve followed the steps here?: https://docs.generatepress.com/article/adding-local-fonts/
Specifically, were you able to do this too?: https://docs.generatepress.com/article/adding-local-fonts/#using-font-face
Kindly let us know how it goes. Hope to hear from you soon. 🙂