If you did not install Gempages app then you can install it from here:
You can find the widget inside Gempages visual editor when you customize a page, you then need to click on Library > Elements, find the widget for Ryviu, click on it and then click Add element button.
After that, you can see Ryviu widget on the left of your visual editor. Now, you can easily add Ryviu into your page by drag and drop it then select widget is Reviews (display comments) or Star Badge (display review star).
Note: Please use Ryviu in your Gempage > Product page > create Product template section