After add Testimonial in gutenberg editor you can switch between 2 mode : Preview mode (Testimonial show as frontend) and Edit Mode (Use for edit testimonials).
To Switch between 2 mode , you click to Testimonial area rounded border (1) a frame border will appear you click to edit icon (2) to switch mode.
With Edit Mode (edit icon is actived) you can edit each Testimonial.
With Preview Mode, you can preview it show as frontend. Switch between 2 mode when working on gutenberg will help you create Testimonial like you want.