Hi there,
GB’s focus is on providing you the tools with to build any type of content.
You could simply recreate the Testimonials Block with a Container, Grid, Headlines ( and the core Image block ).
You can see an example of that here in one of our GB Sites:
https://generatepress.com/site-library/#coach
Once you have one created, you can simply duplicate it or save it as re-useable block.
In GB Pro we will be implementing a Templates library which will come with pre-build templates and allow you to save your own.