I can't seem to find this: what's the best practice for adding Schema markup?

GP adds basic schema markup for you.

However, if you need more advanced markup, I suggest using a plugin like this: https://wordpress.org/plugins/wp-structuring-markup/

Thanks Tom :) Does GP add the star review schema?

It doesn't - that's definitely when you would need a plugin :)

Cool cool. Thanks dad Tom.

Haha no problem :)