The one thing to know about Google Sites
Google Sites places custom code inside an embedded frame. Give the frame enough height for the selected layout so reviews are not cropped.
Where reviews belong on a Google Sites site
Place a wide embed in its own section for a carousel or grid. A smaller frame works for a badge beside contact or location information.
Full-width sections suit carousel and grid layouts; narrow areas suit a badge.
Reviews and search results
The widget outputs review schema markup automatically. Whether Google shows a star rating depends on what the reviews are attached to — worth reading before you expect stars, because for a business reviewing itself the answer is usually no.
What does help either way: reviews on the page are content, and a page that answers what customers actually say is a page people stay on.