Hi there,
GP is built for Mobile Responsiveness. Our code is so lightweight there would be no benefit in restructuring the CSS to load Mobile First.
This article explains the changes 3.0 brings:
https://generatepress.com/generatepress-3-0-a-new-era/
Scroll down to the part titled Flexbox Grid.
You will see that only 1 stylesheet is added by default ie. main.css
Other CSS styles sheets are for components that are conditiionally loaded – if you don’t use a component such as comments or top bar then the CSS for it is not loaded.
The rest of the CSS is dynamic inline or saved to external file – this covers the changes made in the Customizer.
GenerateBlocks external file is also dynamically generated. It is not something to be altered, GB only generates the CSS required for the blocks you have on that page.