Go to Groundplan Theme Settings >> Groundplan Theme Settings.
Turn on "Enable Custom Font Header Package."
Set the Font Source as either Adobe Typekit or Google Font.
- Search Typekit Fonts: https://fonts.adobe.com/fonts
- Search Google Fonts: https://fonts.google.com/
If a Google Font, enter the Font Embed URL.
Under "Font Family Name," enter just the font family name as indicated in the CSS rules. Don't enter the rest of the surrounding CSS.
Hit Update.