Hi there,
currently there are no Tablet typography settings in the customizer – this is under review for a future update.
We can help with some CSS for that – you can see the @media queries that match the theme settings here:
https://docs.generatepress.com/article/responsive-display/#responsive-breakpoints
Let us know if you need help with that.
H1 Heading sets the global style.
The Single Content Title (H1) allows you to override this and set a different style for the Heading used in the Single Post.
Likewise you will see a Heading H2 ( for global styles ) and Archive Content Title (H2) for the post archives heading.