Hi there,
The future of block based themes is unclear especially in regards to how WP will handle the transition of traditional themes to block base templates.
However, we have already begun making significant changes that will allow greater control of how the theme renders templates, which could include Block templates. In 2.5 we make it easier to swap out content templates and GPP 1.11 will introduce Block Elements.
So ideally yes, GP will provide block based styling of the theme either in our way of doing it or hopefully the future core way of doing it.