Google Fonts get automatically loaded once the font has been officially applied to a Content Layer.
To apply a Google Font to a Layer, first edit a content Layers for one of your slides:
Then select a Google Font from the Style Editor:
Then after selecting any Google Font, it will show up at the top of the fonts-list drop-down for easy future access.
And then from the slider’s main settings page, the new Google Fonts section is just a reflection of what’s currently assigned to any of the slide’s Text Layers.
Select a slide from the “Used in Slide” dropdown to be take to the slide editor where the font is being used.
And choose to load the “Latin” charset or not for the font.